Molybdenum Dichalcogenides for Environmental Chemical Sensing

materials

Review Molybdenum Dichalcogenides for Environmental Chemical Sensing

Dario Zappa

Sensor Laboratory, Department of Information Engineering (DII), Università degli Studi di Brescia, Via Valotti 7, 25123 Brescia, Italy; [email protected]; Tel.: +39-030-371-5767

Received: 17 November 2017; Accepted: 5 December 2017; Published: 12 December 2017

Abstract: 2D transition metal dichalcogenides are attracting a strong interest following the popularity of graphene and other carbon-based materials. In the ﬁeld of chemical sensors, they offer some interesting features that could potentially overcome the limitation of graphene and metal oxides, such as the possibility of operating at room temperature. Molybdenum-based dichalcogenides in particular are among the most studied materials, thanks to their facile preparation techniques and promising performances. The present review summarizes the advances in the exploitation of these MoX2 materials as chemical sensors for the detection of typical environmental pollutants, such as NO2, NH3, CO and volatile organic compounds.

Keywords: transition metal dichalcogenides; chemical sensors; air quality; molybdenum dichalcogenides; molybdenum sulﬁde

1. Introduction Transition metal dichalcogenides (TMDs) are a very recent class of materials that are attracting brand new interest in the scientific community. Thanks to the popularity of nanosized carbon-based materials, especially carbon nanotubes (CNTs) and graphene [1–3], many efforts have been spent in the past years on exploring materials which can be easily downsized to 1D and 2D configurations. Metal oxide nanowires, nanoflakes and nanotubes, as well as core–shell and other fancy heterostructures [4–6], have been fabricated with the intent of enhancing the performance of their respective bulk materials. Among 2D structures, TMDs are becoming very popular due to their abundance and very easy preparation techniques. TMDs can be easily described by the chemical formula MX2, where M is a transition metal from groups 4–10 of periodic table (such as Mo, W and V) and X is a chalcogen element (S, Se and Te). In particular, TMDs composed by elements highlighted in Figure1a have the peculiar property to crystallize in ultrathin layers, leading to the formation of single-layered 2D materials. Therefore, TMDs share some structural similarities with graphene, but they also exhibit some complementary properties and features, making them more appealing from application point-of-view. A typical example is the fabrication of electronic transistors: although graphene has remarkably high carrier mobility at room temperature (more than 15,000 cm2/V·s [3]), it has a poorly-defined bandgap, thus it is difficult to turn the transistor to off state. Clearly, it is not well suited to fabricate logic devices in its pristine form. On the contrary, many TMDs are semiconductors, such as MoS2, MoTe2 and WS2; have a wide range of possible bandgaps; and are better suited for their use as an electronic device. According to SCOPUS data, starting from 2012, there was a huge increase in the total number of TMD-related publications, probably due to the “graphene effect” of 2010 Nobel prize [8] that has shifted the scientific focus towards 2D ultrathin materials (Figure1b, SCOPUS data, Elsevier B.V.). Nevertheless, the exploitation of TMDs for the manufacturing of sensor devices is still almost unexplored. According to the data, only less than 4% of total TMDs documents indexed by SCOPUS

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Materials 2017, 10, 1418 2 of 22

database reports sensor applications based on these materials. However, the trend is positive, so it is reasonable to expect an increase of sensor exploitation as soon as the study of these materials

goesMaterials further. 2017, 10, 1418 2 of 22

(a) (b)

Figure 1. (a) Periodic table with highlighted the transition metals and chalcogen elements (S, Se and Te) that form crystalline in 2D layered structures. Co, Rh, Ir and Ni are partially highlighted because only some dichalcogenides form layered structures. Reprinted by permission from Macmillan Publishers Ltd: Nature Chemistry [7], copyright (2013). (b) Number of publications per year on TMDs (in blue) and on TMD-related sensing devices (in red), calculated from fully highlighted materials reported in Figure 1a. * The 2017 data are partial (source SCOPUS, Elsevier B.V.). (a) (b) According to SCOPUS data, starting from 2012, there was a huge increase in the total number of Figure 1. (a) Periodic table with highlighted the transition metals and chalcogen elements (S, Se and TMD-relatedFigure 1. publications,(a) Periodic table probably with highlighted due to thethe transition “graphene metals effect” and chalcogenof 2010 Nobel elements prize (S, Se [8] and that Te) has Te) that form crystalline in 2D layered structures. Co, Rh, Ir and Ni are partially highlighted because that form crystalline in 2D layered structures. Co, Rh, Ir and Ni are partially highlighted because only shiftedonly the some scientific dichalcogenides focus towards form 2D layered ultrathin struct materialsures. Reprinted (Figure by 1b, permission SCOPUS data,from ElsevierMacmillan B.V.). some dichalcogenides form layered structures. Reprinted by permission from Macmillan Publishers Ltd.: Nevertheless,Publishers the Ltd: exploitationNature Chem istryof TMDs [7], copyright for the (2013). manufacturing (b) Number ofof publications sensor devices per year is on still TMDs almost Nature Chemistry [7], copyright (2013); (b) Number of publications per year on TMDs (in blue) and on unexplored.(in blue) According and on TMD-related to the data, sensing only lessdevices than (in 4% red), of totalcalculated TMDs from documents fully highlighted indexed materials by SCOPUS TMD-related sensing devices (in red), calculated from fully highlighted materials reported in Figure1a. databasereported reports in Figure sensor 1a. applications * The 2017 data based are partialon these (source materials. SCOPUS, However, Elsevier theB.V.). tren d is positive, so it is * The 2017 data are partial (source SCOPUS, Elsevier B.V.: Amsterdam, The Netherlands). reasonable to expect an increase of sensor exploitation as soon as the study of these materials goes further.According to SCOPUS data, starting from 2012, there was a huge increase in the total number of Looking at the data in detail, it resulted that scientiﬁc research is mainly focused on transition TMD-relatedLooking atpublications, the data in detail,probably it resulted due to thethat “graphene scientific research effect” of is 2010mainly Nobel focused prize on [8] transition that has metal disulﬁdes (MS ) (Figure2a). This is not surprising: MoS and WS are by far the most investigated metalshifted disulfides the scientific (M2S focus2) (Figure towards 2a). 2DThis ultrathin is not materialssurprising: (Figure2 MoS 21b, and2 SCOPUS WS2 are data, by Elsevierfar the mostB.V.). TMDs, with MoS alone responsible of more than half of total transition metal dichalcogenides investigatedNevertheless, TMDs, the 2exploitation with MoS 2of alone TMDs responsible for the ma ofnufacturing more than of halfsensor of totaldevices transition is still almostmetal publications. Overall, molybdenum dichalcogenides (MoX ) are the most studied group, followed by dichalcogenidesunexplored. According publications. to the data,Overal onlyl, molybdenum less than 4% dichalcogenidesof total2 TMDs documents (MoX2) are indexed the most by SCOPUSstudied tungsten-based ones, as reported in Figure2b. group,database followed reports by sensor tungsten-based applications on basedes, as onreported these ma in terials.Figure 2b.However, the trend is positive, so it is reasonable to expect an increase of sensor exploitation as soon as the study of these materials goes further. Looking at the data in detail, it resulted that scientific research is mainly focused on transition metal disulfides (MS2) (Figure 2a). This is not surprising: MoS2 and WS2 are by far the most investigated TMDs, with MoS2 alone responsible of more than half of total transition metal dichalcogenides publications. Overall, molybdenum dichalcogenides (MoX2) are the most studied group, followed by tungsten-based ones, as reported in Figure 2b.

(a) (b)

Figure 2. (a) Percentage of MS2 (in blue), MSe2 (in red) and MTe2 (in green) manuscripts; and (b) chart Figure 2. (a) Percentage of MS2 (in blue), MSe2 (in red) and MTe2 (in green) manuscripts; and (b) chart reportingreporting the the percentage percentage of of most most common common transition transition metals metals investigated investigated as as TMDs. TMDs. (source (source SCOPUS, SCOPUS, ElsevierElsevier B.V.). B.V.: Amsterdam, The Netherlands).

Among the wide range of existing sensing devices, chemical sensors deserve a special mention. Among the wide range of existing sensing devices, chemical sensors deserve a special mention. These devices can transduce chemical interaction phenomena into a signal that we can manage, These devices can transduce chemical interaction phenomena into a signal that we can manage, compare and evaluate. Gas (asensors) are well-known chemical sensing(b) devices, which may be compare and evaluate. Gas sensors are well-known chemical sensing devices, which may be integrated integrated into personal healthcare (wound monitors, and breath analyzers) and security (toxic into personalFigure 2. (a healthcare) Percentage (wound of MS2 (in monitors, blue), MSe and2 (in red) breath and analyzers) MTe2 (in green) and manuscripts; security (toxic and hazards(b) chart and hazards and explosive detectors) systems. Moreover, they may also be used for environmental explosivereporting detectors) the percentage systems. of most Moreover, common they transition may also metals be investigated used for environmental as TMDs. (source monitoring SCOPUS, and monitoring and for food-chain control. In particular, air pollution is recognized to be one of the most for food-chainElsevier B.V.). control. In particular, air pollution is recognized to be one of the most crucial issues crucial issues for human health, and many efforts have been done by governments to reduce for human health, and many efforts have been done by governments to reduce pollutant emissions. pollutantAmong emissions. the wide The range main of responsible existing sensing of the devices,degradation chemical of air sensors quality deserveare identified a special as CO mention.2, CO, The main responsible of the degradation of air quality are identiﬁed as CO2, CO, NOx, Volatile Organic NOThesex, Volatile devices Organic can transduce Compounds chemical (VOCs), interaction NH3 and phenomena small particulate into a signalmatter that (PM), we i.e., can PM manage,10 and Compounds (VOCs), NH3 and small particulate matter (PM), i.e., PM10 and PM2.5 [9]. The detection of compare and evaluate. Gas sensors are well-known chemical sensing devices, which may be these toxic gases has become more and more essential for our own safety, thus it is necessary to have integrated into personal healthcare (wound monitors, and breath analyzers) and security (toxic hazards and explosive detectors) systems. Moreover, they may also be used for environmental monitoring and for food-chain control. In particular, air pollution is recognized to be one of the most crucial issues for human health, and many efforts have been done by governments to reduce pollutant emissions. The main responsible of the degradation of air quality are identified as CO2, CO, NOx, Volatile Organic Compounds (VOCs), NH3 and small particulate matter (PM), i.e., PM10 and

Materials 2017, 10, 1418 3 of 22 affordable and high performance gas detectors to measure these chemical compounds at sub-ppm level. Traditional gas sensing devices are based on semiconducting oxide materials, and can show excellent performances in terms of sensitivity and long-term stability. However, they are thermally activated and operate at very high temperature (200–600 ◦C), requiring a considerable amount of energy and making their use unsafe in explosive environment [6,10]. In many applications, there is therefore a huge need of sensitive gas sensors that can work at room temperature. Graphene and other carbon-based material are fundamentally and technologically appealing for many applications, including chemical sensing. They may operate at room temperature without requiring a dedicated heating element. However, they are chemically inert and they can only become active and interact with external atmosphere thanks to functionalization with some added molecules [11,12], which in turn results in losing some of the electronic and optical properties. In contrast, 2D TMDs exhibit a versatile chemistry while keeping some graphene features, potentially outperforming the latter in a real sensing environment. However, they are more resistant to chemical functionalization [13], and thus they may suffer the same selectivity issues of metal oxide materials. The goal of the present review is to summarize the advances and applications of molybdenum dichalcogenides as chemical sensors for air quality monitoring, evaluating the advantages and performances in the detection of typical air pollutant such as CO2, CO, NOx, VOCs and NH3.

2. Crystalline Structure and Synthesis Techniques

Molybdenum dichalcogenides, i.e., MoS2, MoSe2 and MoTe2, belong to the large family of layered transition metal dichalcogenides (TMDs) whose crystal structure results from the stacking of sheets of hexagonally packed atoms, with two chalcogen atom planes separated by a plane of metal atoms. Atoms forming this three-layer configuration are strongly packed together by covalent bonds, whereas each three-layer sheet is linked with the next one by Van der Waals bonds, much weaker than covalent bonds. These weak van der Waals forces between the sheets makes it easy to exfoliate thin layers from bulk material. Therefore, they share some properties with graphene, which, unlike TMDs, consists in only a single layer of sp2-bonded carbon atoms in hexagonal configuration [1]. The exfoliation of these materials into mono- or few-layers largely preserves their properties, making them ranging from insulators, semiconductors, true metals and even superconductors at low temperature, such as NbSe2 and TaS2 [7,14]. This peculiar structure leads to a high degree of anisotropy, with different (and usually significantly better) in-plane mechanical, thermal and electronic properties compared to out-of-plane ones [15], ranking for example MoS2 the most anisotropic 2D material after graphite [16]. Crystal phase is not unique for all TMDs: they exhibit a wide range of polymorphs depending on the phase of a single monolayer, which itself contains three layers of atoms (X-M-X), and on how monolayers stack together to form a bulk material. Therefore, a single TMD can be found in many different polymorphs, and its crystal structure is strongly related to its formation history. Within a single monolayer, TMDs can exhibit only two polymorphs, directly related to metal coordination: trigonal prismatic (D3h point group) or octahedral (D3d point group), with a preferred structure depending on the specific combination of chalcogen and transition metal (Figure3). Taken by themselves, these monolayers could be also named as 1H and 1T polymorphs, respectively, where “T” stands for trigonal and “H” stands for hexagonal. The digit refers to the number stacking layers (one in the case of a monolayer), which is also the number of X-M-X units forming the unit cell [7]. Bulk materials, instead, can be found in many different polymorphs. Most common ones are 1T, 2H and 3R (“R” stands for rhombohedral), which can easily be described as stacking sequence of monolayers. For example, 2H is characterized by |AbA BaB| stacking sequence, where capital and lower letters refer to chalcogen and metal atoms, respectively. The 3R crystal structure, instead, has a stacking sequence of |AbA CaC BcB|. Molybdenum dichalcogenides usually crystallize in 2H structure. Trigonal prismatic metal coordination is the most energetic favorable structure, and is the reason for the semiconducting Materials 2017, 10, 1418 4 of 22

behavior of these materials [17]. In some cases, synthetic MoS2 and MoSe2 could also crystallize as 3R ◦ triangular prismatic. For MoTe2, we have a phase transition at temperature higher than 815 C from semiconducting 2H α-MoTe2 to metallic β-MoTe2, exhibiting a monoclinic structure with distorted octahedralMaterials 2017 coordination, 10, 1418 [18,19]. 4 of 22

Figure 3. TrigonalTrigonal prismatic ( a); and octahedral ( b) metal metal coordination, coordination, with respective c-axis and side

sections, for MoX2 materials.materials. Mo Mo atoms atoms are are in in blue, blue, chalcogenides chalcogenides (X) atoms in yellow. ( a) Represents a monolayer with 1H crystal structure, while (b) a monolayer with 1T crystal structure. Atomic radii are not in scale.

Bulk materials, instead, can be found in many different polymorphs. Most common ones are 1T, A generic MoX2 unit cell in 2H polymorph is displayed in Figure4. A molybdenum atom plane 2H and 3R (“R” stands for rhombohedral), which can easily be described as stacking sequence of is between two chalcogen planes, forming a monolayer. Two stacked layers are displaced respect to monolayers. For example, 2H is characterized by |AbA BaB| stacking sequence, where capital and each other, having the metal atoms of the ﬁrst layer directly above (along c-axis) the chalcogenides lower letters refer to chalcogen and metal atoms, respectively. The 3R crystal structure, instead, has atoms of the second one, and vice versa. As previously described, layers are kept together by Van der a stacking sequence of |AbA CaC BcB|. Waals forces. The electronic structure of TMDs strongly depends on the coordination of the transition Molybdenum dichalcogenides usually crystallize in 2H structure. Trigonal prismatic metal metal and the number of electrons in the d-orbital: for trigonal coordinated molybdenum, orbitals coordination is the most energetic favorable structure, and is the reason for the semiconducting are fully occupied and Mo-dichalcogenides materials are thus semiconductors. Chalcogenides atoms, behavior of these materials [17]. In some cases, synthetic MoS2 and MoSe2 could also crystallize as 3R instead, have a minor effect on electronic properties. Lattice parameters increase with the increase of triangular prismatic. For MoTe2, we have a phase transition at temperature higher than 815 °C from atomic number of the chalcogen, making the unit cell bigger, as reported in Table1. At the same time, semiconducting 2H α-MoTe2 to metallic β-MoTe2, exhibiting a monoclinic structure with distorted we can observe a gradual reduction of the indirect bandgap, for instance, due to the broadening of octahedral coordination [18,19]. d-bands [20]. A generic MoX2 unit cell in 2H polymorph is displayed in Figure 4. A molybdenum atom plane isTable between 1. Cell two and chalcogen structural parametersplanes, forming and measured a monolayer. bandgaps Two of 2Hstacked polytype layers Mo are dichalcogenides displaced respect [18–20 ].to each other, having the metal atoms of the first layer directly above (along c-axis) the chalcogenides atoms of the second one, and vice versa. As previouslyMoS described,2 MoSe2 layersMoTe are2 kept together by Van der Waals forces. The electronic structurae [Å] of TMDs strongly3.160 depends 3.299 on the 3.522 coordination of the transition metal and the number of electrons inc [Å]the d-orbital: for12.294 trigonal 12.938 coordina 13.968ted molybdenum, orbitals are fully occupied and Mo-dichalcogenides2z [Å] materials 3.172are thus 3.338semiconductors. 3.604 Chalcogenides atoms, instead, have a minor effect on electronicw [Å] properties2.975. Lattice 3.131parameters 3.380 increase with the increase of atomic number of the chalcogen, makingc/a [Å] the unit cell3.891 bigger, 3.922as reported 3.966 in Table 1. At the same time, Indirect Bandgap [eV] 1.29 1.10 1.00 we can observe a gradual reductionDirect Bandgap of the [eV] indirect 1.78 bandgap, 1.42 for instance, 1.00 due to the broadening of d-bands [20]. Electronic and optical properties not only depend on their chemical composition, but also on the thickness of these materials, and can be dramatically different. For example, bulk MoS2 shows an indirect bandgap of ≈1.3 eV, as reported in Table 1. However, an isolated MoS2 monolayer is a semiconductor exhibiting a direct bandgap of ≈1.8 eV due to quantum confinement effects, and thus enhancing significantly the photoluminescence compared to bulk material [21,22].

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Figure 4. Crystal structure of 2H structured Mo dichalcogenides, withwith |AbA|AbA BaB| stacking sequence, wherewhere capitalcapital andand lowerlower lettersletters referrefer toto chalcogenchalcogen andand metalmetal atoms,atoms, respectively.respectively.

Table 1. Cell and structural parameters and measured bandgaps of 2H polytype Mo dichalcogenides Electronic and optical properties not only depend on their chemical composition, but also on [18–20]. the thickness of these materials, and can be dramatically different. For example, bulk MoS2 shows an indirect bandgap of ≈1.3 eV, as reported inMoS Table2 1. However,MoSe2 an isolatedMoTe2 MoS 2 monolayer is a semiconductor exhibiting aa direct [Å] bandgap of ≈ 3.1601.8 eV due to 3.299 quantum confinement 3.522 effects, and thus enhancing significantly the photoluminescencec [Å] 12.294 compared to 12.938 bulk material 13.968 [21,22 ]. Quite interestingly, it is2z possible [Å] to intercalate 3.172 alkali metals 3.338 to induce a 3.604 phase change in Mo-based TMDs. For example, we canw turn[Å] semiconducting 2.975 2H-MoS 2 3.131into metallic 3.380 1T-MoS 2 [23] by lithium or potassium intercalationc/a [13 [Å],24– 26], even if 3.891 the 1T phase 3.922 is not thermodynamically 3.966 stable and switch back to the originalIndirect Bandgap 2H polymorph [eV] over 1.29 time, even at 1.10 room temperature 1.00 [27]. Local phase transformations couldDirect potentially Bandgap lead [eV] to hybrid 1.78 metal-semiconductor 1.42 1T-2H, 1.00 representing unique heterojunctions over a single homogeneous layer. TheQuite identification interestingly, of it crystallographic is possible to intercalate phases of MoalkaliX2 metalscompounds to induce could a be phase done change by using in standardMo-based spectroscopic TMDs. For example, techniques we [ 28can]. Figureturn semiconducting5 shows X-ray Spectroscopy 2H-MoS2 into (XRD) metallic peaks 1T-MoS of 2H2 and[23] 1Tby MoSlithium2 (Figure or potassium5a) and MoSeintercalation2 (Figure [13,24–26],5b). Thespectrum even if the of 1T 2H phase MoS 2is, fornot example, thermodynamically shows an intense stable peakand switch at 14◦ relatedback to tothe (002) original plane 2H (ICSD polymorph code: 84183), over time, indicating even at a droom-spacing temperature of ≈6.2 Å [27]. in line Local with phase cell parameterstransformations reported could in Tablepotentially1. In Li-intercalated lead to hybrid 1T metal-semiconductor structure, instead, (002) 1T-2H, peaks representing is almost neglected, unique whileheterojunctions we observe over a new a single (001) homogeneous reflection at ≈ 8.5layer.◦. X-rayThe identification Photoelectron of Spectroscopy crystallographic (XPS) phases is another of techniqueMoX2 compounds for analyzing could in detailbe done the by chemical using statestandard of 2H spectroscopic and 1T phases. techniques In Figure [28].5 are Figure reported 5 shows fine X-ray XPS Spectroscopy spectra of Mo (XRD) 3d and peaks S 2p of for 2H both and 2H1T MoS and2 1T (Figure phases 5a) of and MoS MoSe2 (Figure2 (Figure5c,d) 5b). and The MoSe spectrum2 (Figure of 2H5e,f). MoS By2,deconvolution for example, shows of the an peaks intense is possiblepeak at 14° to distinguishrelated to (002) the contributionplane (ICSD ofcode: both 84183), phases, indicating estimating a d also-spacing relative of ≈ concentrations6.2 Å in line with [29 ,cell30]. Besides,parameters Raman reported spectroscopy in Table can 1. easily In Li-intercalated identify the dichalcogenides 1T structure, polymorphs,instead, (002) but peaks cannot is providealmost accurateneglected, quantitative while we observe analysis. a new For (001) example, reflection 1T phase at ≈8.5°. have symmetry differences which results in severalX-ray additional Photoelectron vibration Spectroscopy modes (J1, (XPS)J2 and isJ 3another) not active technique in 2H for (Figure analyzing5g for in MoS detail2 and the Figure chemical5h forstate MoSe of 2H2)[ and26,27 1T,30 phases.]. In Figure 5 are reported fine XPS spectra of Mo 3d and S 2p for both 2H and 1TThe phases chemical of MoS composition,2 (Figure 5c,d) phase and MoSe structure,2 (Figure crystal 5e,f). By quality, deconvolut numberion of of the layers peaks andis possible edge morphologiesto distinguish the have contribution a strong effect of both on thephases, performances estimating of also molybdenum relative concentrations dichalcogenides. [29,30]. However, Besides, theRaman requirements spectroscopy depend can on easily the proposed identify application:the dichalcogenides for high-end polymorp electronichs, devicesbut cannot it is necessary provide toaccurate fabricate quantitative high-purity analysis. and dopant-free For example, materials, 1T phase while have for solar symmetry industry differences manufacturing which cost results is a key in feature,several additional and thus it vibration is acceptable modes to ( haveJ1, J2 and a certain J3) not amount active ofin 2H defects (Figure in the 5g materialfor MoS2 [ 31and]. ChemicalFigure 5h sensingfor MoSe performances,2) [26,27,30]. in particular, are strongly affected by the synthesis technique used and the fabrication history of the samples. Across the years, many techniques have been developed for the fabrication of bulk and 2D thin film TMDs materials, which can be classified mainly as top-down and bottom-up approaches (Figure6).

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Figure 5. XRD pattern of: 1T and 2H MoS2 (a); and MoSe2 (b). XPS results for: Mo 3d (c); and S 2p (d)

peaks of pure 2H and Li-intercalated MoS2. XPS results for: Mo 3d (e); and S 2p (f) peaks of pure 2H and Li-intercalated MoSe2. Raman spectra of: 1T and 2H MoS2 (g); and MoSe2 (h). Reprinted (adapted) with permission from [28]. Copyright (2016) American Chemical Society.

The chemical composition, phase structure, crystal quality, number of layers and edge morphologies have a strong effect on the performances of molybdenum dichalcogenides. However, the requirements depend on the proposed application: for high-end electronic devices it is necessary to fabricate high-purity and dopant-free materials, while for solar industry manufacturing cost is a key feature, and thus it is acceptable to have a certain amount of defects in the material [31]. Chemical Figure 5. XRD pattern of: 1T and 2H MoS2 (a); and MoSe2 (b). XPS results for: Mo 3d (c); and S 2p (d) sensingFigure performances, 5. XRD pattern in of:particular, 1T and 2H are MoS strongly2 (a); and affected MoSe2 ( bby). XPSthe resultssynthesis for: Motechnique 3d (c); and used S 2p and (d) the peaks of pure 2H and Li-intercalated MoS2. XPS results for: Mo 3d (e); and S 2p (f) peaks of pure 2H fabricationpeaks ofhistory pure 2H of andthe Li-intercalatedsamples. Across MoS the2. XPS years, results many for: Motechniques 3d (e); and have S 2p been (f) peaks developed of pure 2Hfor the and Li-intercalated MoSe2. Raman spectra of: 1T and 2H MoS2 (g); and MoSe2 (h). Reprinted (adapted) fabricationand Li-intercalated of bulk and MoSe2D thin2. Raman film TMDs spectra materials, of: 1T and 2Hwhich MoS can2 (g); be and classified MoSe2 (h mainly). Reprinted as top-down (adapted) and bottom-upwithwith permissionpermission approaches fromfrom (Figure [[28].28]. CopyrightCopyright 6). (2016)(2016) AmericanAmerican ChemicalChemical Society:Society. Washington, DC, USA.

Mechanical exfoliation was the earliest method introduced, back in the 1960s, to obtain 2D MoS2 Mechanical exfoliation was the earliest method introduced, back in the 1960s, to obtain 2D [32]. It mainly consists in the removal of thin layers from the parent solid bulk thanks to subsequent MoS2 [32]. It mainly consists in the removal of thin layers from the parent solid bulk thanks to Scotch adhesive tape transfers. Afterwards, the tape is placed on a target substrate and removed, subsequent Scotch adhesive tape transfers. Afterwards, the tape is placed on a target substrate and leaving very thin layers (eventually monolayers) of materials. Although this technique is extremely removed, leaving very thin layers (eventually monolayers) of materials. Although this technique cheap and could prepare high-quality monolayers, suitable to demonstrate high performances is extremely cheap and could prepare high-quality monolayers, suitable to demonstrate high devices, it has some major drawbacks. Practical applications require fast production scale and bulk performances devices, it has some major drawbacks. Practical applications require fast production quantities of materials, which can hardly be obtained by Scotch tape technique. scale and bulk quantities of materials, which can hardly be obtained by Scotch tape technique. TMDs, i.e., molybdenum dichalcogenides, are composed by uncharged layers kept together by TMDs, i.e., molybdenum dichalcogenides, are composed by uncharged layers kept together Van der FigureWaals 6.forces. Bottom-Up Contrary and Top-Down to charged-sheets approaches solids for the suchsynthesis as perovskites, of thin 2D Mo uncharged-layeredX2 materials. by Van der Waals forces. Contrary to charged-sheets solids such as perovskites, uncharged-layered solids cannot be chemically exfoliated easily [33]. Liquid alkali-atoms intercalation, such as lithium solids cannot be chemically exfoliated easily [33]. Liquid alkali-atoms intercalation, such as lithium for example,Mechanical enables exfoliation the exfoliation was the earliestof few-layers method TMDs introduced, and eventually back in the monolayers, 1960s, to obtain and 2Dis more MoS 2 for example, enables the exfoliation of few-layers TMDs and eventually monolayers, and is more effective[32]. It mainly for devices consists mass in theproduction. removal ofBy thin adding layers some from lithium-based the parent solid compounds, bulk thanks such to subsequentas n-butyl effectiveScotch adhesive for devices tape mass transfers. production. Afterwards, By adding the tape some is lithium-basedplaced on a target compounds, substrate such andas removed,n-butyl lithium, in hexane solution it is possible to insert lithium atoms between every MoX2 layer, as an lithium,leaving very in hexane thin layers solution (eventually it is possible monolayers) to insert of lithium materials. atoms Although between this every technique MoX2 islayer, extremely as an intercalation agent. The reaction results in the formation of an intermediate LixMoX2 solid [34,35]. intercalation agent. The reaction results in the formation of an intermediate LixMoX2 solid [34,35]. Layerscheap areand then could separated prepare by high-quality simple sonication monolayers, in water, suitable whereas to lithiumdemonstrate atoms highare detached performances from Layersdevices, are it thenhas some separated major by drawbacks. simple sonication Practical in applications water, whereas require lithium fast atoms production are detached scale and from bulk the the layers. Figure 7a reports an example of a typical liquid lithium exfoliation process for MoS2 layers. Figure7a reports an example of a typical liquid lithium exfoliation process for MoS 2 [36,37]. quantities of materials, which can hardly be obtained by Scotch tape technique. This veryTMDs, effective i.e., molybdenum technique can dichalcogenides, produce very high are yieldcomposed of monolayers, by uncharged almost layers close kept to 100% together rate [by7]. However,Van der Waals it requires forces. very Contrary long time to (>3 charged-sheets days) and accurate solids control such ofas the perovskites, process to avoiduncharged-layered the formation ofsolids undesired cannot metalbe chemically nanoparticles exfoliated such easily as Li2 S.[33]. Moreover, Liquid alkali-atoms lithium exfoliation intercalation, could leadsuch toas alithium phase changesfor example, from enables semiconducting the exfoliation 2H to of metallic few-layers 1T, alteringTMDs and the eventually electronic andmonolayers, optical properties and is more of originaleffective molybdenum for devices mass dichalcogenides production. materialsBy adding [25 some,28,38 lithium-based], which can be compounds, restored afterwards such as byn-butyl heat treatmentslithium, in[ 39hexane]. An alkali-freesolution it alternative is possible has to beeninsert recently lithium proposed atoms between by Coleman every et al.Mo [X402], layer, combining as an theintercalation advantages agent. of liquid The reaction sonication-assisted results in the exfoliation formation without of an intermediate causing distortions LixMoX2 to solid the [34,35]. crystal Layers are then separated by simple sonication in water, whereas lithium atoms are detached from the layers. Figure 7a reports an example of a typical liquid lithium exfoliation process for MoS2

Materials 2017, 10, 1418 7 of 22

[36,37]. This very effective technique can produce very high yield of monolayers, almost close to 100% rate [7]. However, it requires very long time (>3 days) and accurate control of the process to avoid the formation of undesired metal nanoparticles such as Li2S. Moreover, lithium exfoliation could lead to a phase changes from semiconducting 2H to metallic 1T, altering the electronic and optical properties of original molybdenum dichalcogenides materials [25,28,38], which can be restored

Materialsafterwards2017, 10 by, 1418 heat treatments [39]. An alkali-free alternative has been recently proposed7 of 22by Coleman et al. [40], combining the advantages of liquid sonication-assisted exfoliation without causing distortions to the crystal structure. This latter technique was successfully employed to structure.fabricate Thisthin latterlayers technique of molybdenum was successfully dichalcogenides; employed however, to fabricate it has thin a much layers lower of molybdenum yield in the dichalcogenides;preparation of monolayers. however, it has a much lower yield in the preparation of monolayers.

(a) (b)

Figure 7. (a) Liquid exfoliation of MoS2 by using three different compounds: methyl lithium, n-butyl Figure 7. (a) Liquid exfoliation of MoS2 by using three different compounds: methyl lithium, n-butyl lithium and tert-butyl lithium. Reprinted with permission from [37]. Copyright (2015) Wiley-VCH lithium and tert-butyl lithium. Reprinted with permission from [37]. Copyright (2015) Wiley-VCH Verlag GmbH. (b) Schematic of conversion process from MoO3 layer to MoS2. Reprinted with Verlag GmbH: Hoboken, NJ, USA; (b) Schematic of conversion process from MoO3 layer to MoS2. Reprintedpermission with from permission [41]. Copyright from [41 (2012)]. Copyright The Royal (2012) Society The Royal of Chemistry. Society of Chemistry: London, UK.

Exfoliation methods produce quasi-2D materials, but require a certain amount of solid bulk Exfoliation methods produce quasi-2D materials, but require a certain amount of solid bulk crystals as source material, which have to be synthetized separately. Common techniques to prepare crystals as source material, which have to be synthetized separately. Common techniques to TMDs crystals are vapor-phase techniques such as chemical vapor transport (CVT) [42,43] and prepare TMDs crystals are vapor-phase techniques such as chemical vapor transport (CVT) [42,43] powder vaporization [44,45], mainly based on an evaporation-condensation process in a controlled andenvironment. powder vaporization Other techniques [44,45 ],used mainly for basedthe pr oneparation an evaporation-condensation of 2D thin films of molybdenum process in adichalcogenides controlled environment. include Othermolecular techniques beam usedepitaxy for (Van the preparation der Waals ofEpitaxy 2D thin (VDWE)) films of molybdenum [46,47], metal dichalcogenidesorganic chemical include vapor molecular deposition beam (MOCVD) epitaxy (Van [48, der49] Waals and Epitaxydirect metal (VDWE)) or [oxide46,47], conversion metal organic via chemicalexposure vapor to a chalcogen deposition vapor (MOCVD) [41,50]. [48 Figure,49] and 7b re directpresents metal a schematic or oxide conversionof the sulfurization via exposure of a thin to a chalcogen vapor [41,50]. Figure7b represents a schematic of the sulfurization of a thin MoO 3 layer to MoO3 layer to obtain MoS2 on sapphire substrates [41]. A thin layer of desired thickness was obtaindeposited MoS 2byon thermal sapphire evaporation substrates of [41 molybdenum]. A thin layer ox ofide desired powder thickness on top of was c-face deposited sapphire, by thermaland then evaporationannealed in ofthe molybdenum furnace. Afterwards, oxide powder samples on were top heated of c-face at high sapphire, temperature and then together annealed with a in source the furnace.of sulfur Afterwards, in inert atmosphere, samples wereresulting heated in oxide-dichalcogen at high temperature conversion. together withThis technique a source of is sulfurvery easy in inertand atmosphere,leads to continuous resulting TDMs in oxide-dichalcogen films; however, it conversion. often results This in undesired technique nanocrystalline is very easy and structures. leads to continuous TDMs ﬁlms; however, it often results in undesired nanocrystalline structures.

3. Molybdenum Disulfide (MoS2) Chemical Sensors 3. Molybdenum Disulfide (MoS2) Chemical Sensors MoS2 is by far the most studied 2D material after graphene, and it could be considered as a MoS2 is by far the most studied 2D material after graphene, and it could be considered as prototypal TMD. MoS2 is an n-type semiconductor with highest direct and indirect bandgaps a prototypal TMD. MoS is an n-type semiconductor with highest direct and indirect bandgaps compared to other molybdenum2 dichalcogenides (Table 1). It has attracted huge attention in the last compared to other molybdenum dichalcogenides (Table1). It has attracted huge attention in the last few years because of its excellent nanoelectronic, optoelectronic, and energy harvesting properties. few years because of its excellent nanoelectronic, optoelectronic, and energy harvesting properties. Therefore, many research groups have started investigating the chemical sensing performances of Therefore, many research groups have started investigating the chemical sensing performances of devices based on this interesting material [51]. devices based on this interesting material [51]. Especially suited for the fabrication of biosensors [52,53], MoS2 is attracting a strong interest in Especially suited for the fabrication of biosensors [52,53], MoS2 is attracting a strong interest the field of chemical sensors for the detection on nitrogen dioxide (NO2), ammonia (NH3) and ethanol, in the field of chemical sensors for the detection on nitrogen dioxide (NO2), ammonia (NH3) and ethanol, among the most common pollutant gases. Nevertheless, Perkins et al. [54–56] investigated in detail the sensing properties of CVD MoS2 monolayers toward some laboratory chemicals and solvents, including triethylamine (TEA). Both simple conductometric [54] and FET [55] devices proved to be very sensitive to TEA and acetone, exhibiting almost no response to many other chemicals such as dimethylmethylphosphate (DMMP). In 2017, Li et al. [57] further improved TEA detection Materials 2017, 10, 1418 8 of 22 among the most common pollutant gases. Nevertheless, Perkins et al. [54–56] investigated in detail the sensing properties of CVD MoS2 monolayers toward some laboratory chemicals and solvents, including triethylamine (TEA). Both simple conductometric [54] and FET [55] devices proved to be very sensitive to TEA and acetone, exhibiting almost no response to many other chemicals such as Materials 2017, 10, 1418 8 of 22 dimethylmethylphosphate (DMMP). In 2017, Li et al. [57] further improved TEA detection by fabricating a core–shell heterostructure, described as Au@SnO2/MoS2, by using a combination of bydifferent fabricating techniques. a core–shell heterostructure, described as Au@SnO2/MoS2, by using a combination of differentCho techniques.et al. [58,59] were among the first proposing CVD grown MoS2 for gas sensing applications. In particular,Cho et al. systematic [58,59] were pressure among control the first during proposing the CVD CVD process grown MoS resulted2 for gasin highly sensing uniform applications. three- Inlayer particular, MoS2 films systematic on 2” pressure wafer controlscale. Resistance during the responses CVD process (∆R/R) resulted were in highlyinvestigated uniform toward three-layer two MoScommon2 films polluting on 2” wafer gases: scale. NO2 Resistanceand NH3, at responses concentrations (∆R/R) from were 1.2 investigated to 50 ppm (Figure toward 8). two In particular, common pollutingsensor resistance gases: NO increased2 and NH in3 ,presence at concentrations of NO2 gas: from NO 1.22 toacts 50 as ppm an (Figureelectron8). acceptor, In particular, resulting sensor in resistancep-doping of increased the material. in presence On the of contrary, NO2 gas: the NO resistance2 acts as an of electron the MoS acceptor,2 sensing resulting device decreased in p-doping with of thethe material.adsorption On of the NH contrary,3 gas molecules. the resistance In Fact, of the NH MoS3 acts2 sensing as an electron device decreaseddonor (i.e., with n-doping) the adsorption shifting ofthe NH Fermi3 gas level molecules. of the InMoS Fact,2 closer NH3 actsto the as conduction-band an electron donor edge. (i.e., n -doping)First-principles shifting de thensity Fermi functional level of thetheory MoS calculations2 closer to the indicated conduction-band that NO edge.2 and First-principlesNH3 molecules density have functionalnegative adsorption theory calculations energies indicated(i.e., the adsorption that NO2 processesand NH3 aremolecules exothermic). have Thus, negative NO adsorption2 and NH3 molecules energies(i.e., are likely the adsorption to adsorb processesonto the surface are exothermic). of the MoS Thus,2. Complete NO2 and recovery NH3 moleculesof the baseline are likelywas hard to adsorb to achieve: onto theincreasing surface the of theworking MoS2 .temperature Complete recovery to 100 °C of thesped baseline up the was recovery hard to substantially achieve: increasing (Figure the 8c). working The charge temperature transfer ◦ tomechanism 100 C sped between up the the recovery gas molecules substantially and MoS (Figure2 was 8validatedc). The charge by theoretical transfer calculations, mechanism indicating between thethat gas the molecules Fermi-level and shift MoS induced2 was validated by the NH by3 molecules theoretical is calculations, negligible. Interestingly, indicating that if theSiO Fermi-level2 substrates shiftwere induced used instead by the of NH sapphire3 molecules wafers, is negligible. 2D MoS2 material Interestingly, switched if SiO its2 substratessemiconducting were used behavior instead from of sapphiren- to a p-type, wafers, due 2D MoSto doping2 material caused switched by SiO its2 dangling semiconducting oxygen behavior bonds [60]. from Finally,n- to a psame-type, authors due to dopingfabricated caused a bifunctional by SiO2 dangling device able oxygen to work bonds as gas [60 ].sensor Finally, and same as photodetector authors fabricated simultaneously a bifunctional [59]. deviceGas sensing able to workmeasurements as gas sensor showed and as good photodetector response simultaneously to low concentrations [59]. Gas sensingof NO measurements2, although in showednitrogen good atmosphere. response Moreover, to low concentrations the same measuremen of NO2, althoughts under in 650 nitrogen nm light atmosphere. illumination Moreover, resulted the in samelower measurements performances underof the devices. 650 nm light illumination resulted in lower performances of the devices.

Figure 8. (a) Image of MoS2 semi-transparent film on the 2” sapphire substrate. (b) Cross-sectional Figure 8. (a) Image of MoS2 semi-transparent ﬁlm on the 2” sapphire substrate; (b) Cross-sectional Transmission Electron Microscopy (TEM) images, demonstrating that the synthesized MoS2 films Transmission Electron Microscopy (TEM) images, demonstrating that the synthesized MoS2 ﬁlms 2 2 consistedconsisted of three layers of MoS MoS2. ;((cc) )Transient Transient NO NO 2gasgas response response atat 1.5 1.5 to to50 50 ppm ppm concentration, concentration, at atoperating operating temperatures temperatures of ofRT RT and and 100 100 °C.◦C. The The recovery recovery rate rate was was faster faster at at 100 100 °C◦C than than at at RT. RT;

((dd)) ComparisonComparison ofof responsesresponses to to NONO22 andand NHNH33. Reprinted Reprinted with permission from [[58].58]. Copyright (2015) NatureNature PublishingPublishing Group:Group. London, UK.

TheThe possiblepossible reasonreason ofof suchsuch aa phenomenonphenomenon waswas explainedexplained byby LateLate etet al.al. [[61].61]. AuthorsAuthors preparedprepared large-area MoS2 sheets ranging from single to five layers on 300 nm SiO2/Si substrates using the large-area MoS2 sheets ranging from single to five layers on 300 nm SiO2/Si substrates using the micromechanicalmicromechanical exfoliationexfoliation method, method, fabricating fabricating field field effect effect transistor transistor (FET) (FET) sensing sensing devices devices that were that were assessed for gas-sensing performances to NO2, NH3 and humidity exposure, in different assessed for gas-sensing performances to NO2, NH3 and humidity exposure, in different conditions of conditions of gate bias and light irradiation (Figure 9). Single layer devices had stability issues: they gate bias and light irradiation (Figure9). Single layer devices had stability issues: they were not stable were not stable in air, and thus were not discussed. Interestingly, authors noticed that the five-layer in air, and thus were not discussed. Interestingly, authors noticed that the five-layer MoS2 sample MoS2 sample has better sensitivity (∆R/R) compared to that of the two-layer MoS2 sample. This has better sensitivity (∆R/R) compared to that of the two-layer MoS2 sample. This phenomenon phenomenon may be due to the different electronic structures caused by the different number of may be due to the different electronic structures caused by the different number of stacked layers. stacked layers. Electrical resistance in FET MoS2 can be tuned by gate biasing, which makes this Electrical resistance in FET MoS2 can be tuned by gate biasing, which makes this material more material more competitive for gas sensing compared to, e.g., graphene. Thickest MoS2 device was competitive for gas sensing compared to, e.g., graphene. Thickest MoS2 device was more susceptible to the influence of gate bias. For all devices, however, recovery was not complete. To overcome this issue, researcher illuminated the samples with a green (532 nm) LED, instead of similar works UV light that could damage the structure due to the higher photon energy. Low power density irradiation slightly increased the response of devices compared to dark ones, but at higher power density the performances Materials 2017, 10, 1418 9 of 22

more susceptible to the influence of gate bias. For all devices, however, recovery was not complete.

MaterialsTo overcome2017, 10 ,this 1418 issue, researcher illuminated the samples with a green (532 nm) LED, instead9 of 22of similar works UV light that could damage the structure due to the higher photon energy. Low power density irradiation slightly increased the response of devices compared to dark ones, but at higher decreasedpower density significantly, the performances as reported decreased by [59] significantl also. Wheny, there as reported are too by many [59] photocarriersalso. When there generated are too undermany photocarriers strong illumination, generated not allunder the strong excited illuminati electrons/holeson, not reactall the with excited gas molecules.electrons/holes Moreover, react with it is possiblegas molecules. that at highMoreover, power it density is possible the desorption that at high rate power increases density more the than desorption the adsorption rate increases rate because more ofthan the the light-induced adsorption activation rate because under of irradiation. the light-in Theduced humidity activation sensing under performances irradiation. of The two-layer humidity and five-layersensing performances MoS2 sensor of devices two-layer were and also five-layer investigated. MoS Water2 sensor vapor devices is considered were also aninvestigated. electron acceptor Water similarvapor is to considered NO2, so the an resistance electron ofacceptor MoS2 should similar increase to NO2, with so the the resistance relative humidity of MoS2 (RH).should It resultedincrease thatwith belowthe relative RH of humidity 60% devices (RH). were It resulted almost that insensitive below RH to of humidity. 60% devices However, were almost at higher insensitive humidity to levels,humidity. the resistancesHowever, at change higher dramatically, humidity levels, especially the resistances for five-layer change devices. dramatically, Other research especially groups for fabricatingfive-layer devices. MoS2 devices Other byresearch different groups techniques fabricating and withMoS different2 devices morphologiesby different techniques also confirmed and with this importantdifferent morphologies outcome [62– also64]. confirmed this important outcome [62–64].

Figure 9. (a(a) )SEM SEM image image of oftwo-layer two-layer MoS MoS2 transistor2 transistor device. device; (b) Sensing (b) Sensing behavior behavior of five-layer of ﬁve-layer MoS2 MoSexposed2 exposed to 100 toppm 100 NO ppm2 under NO2 under green greenlight illumination. light illumination; (c) Resistance (c) Resistance as a function as a function of RH of for RH two- for two-layerlayer MoS MoS2 samples.2 samples. Reprinted Reprinted (adapted) (adapted) with with permi permissionssion from from [61]. [61]. Copyright Copyright (2013) (2013) American Chemical Society:Society. Washington, DC, USA.

A detailed analysis on a Schottky-contacted CVD monolayer MoS2 FET for the detection of NH3 A detailed analysis on a Schottky-contacted CVD monolayer MoS2 FET for the detection of NH3 and NO2 was reported by Liu et al. [65]. Authors believed that Schottky barrier modulation was the and NO2 was reported by Liu et al. [65]. Authors believed that Schottky barrier modulation was key factor for the significantly improved sensitivity, and that detection limit might be pushed to sub- the key factor for the significantly improved sensitivity, and that detection limit might be pushed to ppb level by optimizing the features of the Schottky barrier. sub-ppb level by optimizing the features of the Schottky barrier. Donarelli et al. [66] fabricated conductometric devices by dispersing liquid-exfoliated MoS2 and Donarelli et al. [66] fabricated conductometric devices by dispersing liquid-exfoliated MoS2 and evaluated the performances toward NO2 in real air environment, investigating the effect of relative evaluated the performances toward NO2 in real air environment, investigating the effect of relative humidity on the response (Rair/Rgas) also. By controlling the annealing temperature, they were able to humidity on the response (Rair/Rgas) also. By controlling the annealing temperature, they were able to force a change in the semiconducting behavior of the material from p-type (150 °C) to n-type (250 °C). force a change in the semiconducting behavior of the material from p-type (150 ◦C) to n-type (250 ◦C). Although more difficult to achieve, chemical functionalization and the fabrication of Although more difficult to achieve, chemical functionalization and the fabrication of heterostructures are effective techniques to tune the performances of functional devices. Lu et al. [67] heterostructures are effective techniques to tune the performances of functional devices. Lu et al. [67] proposed an interesting technique to include Au atoms in the MoS2 lattice. This approach firstly used proposed an interesting technique to include Au atoms in the MoS2 lattice. This approach firstly used a focused laser beam to locally unbound Sulfur atoms. Afterwards, substrates were immersed in a focused laser beam to locally unbound Sulfur atoms. Afterwards, substrates were immersed in AuCl3 solution, forcing the anchoring of Au nanoparticles to these active sites. Samples were then AuCl3 solution, forcing the anchoring of Au nanoparticles to these active sites. Samples were then characterized as starting brick for surface enhanced Raman scattering (SERS) devices. characterized as starting brick for surface enhanced Raman scattering (SERS) devices. A more conservative approach was illustrated by Baek et al. [68]. Authors put Pd nanoparticles A more conservative approach was illustrated by Baek et al. [68]. Authors put Pd nanoparticles by by simple thermal evaporation on top of commercially available MoS2 sheets deposited by drop simple thermal evaporation on top of commercially available MoS2 sheets deposited by drop casting, casting, to fabricate a resistive hydrogen sensors. The thickness of the Pd nanoparticle layer was to fabricate a resistive hydrogen sensors. The thickness of the Pd nanoparticle layer was controlled controlled from 1 nm to 7 nm. At low Pd thickness functionalization resulted in an increase of the from 1 nm to 7 nm. At low Pd thickness functionalization resulted in an increase of the response to 1% response to 1% H2 keeping the same baseline resistance of devices. On the contrary, at highest Pd H2 keeping the same baseline resistance of devices. On the contrary, at highest Pd concentration the concentration the sensing layer suddenly became metallic. The reason is that Pd nanoparticles formed sensing layer suddenly became metallic. The reason is that Pd nanoparticles formed a continuous film, a continuous film, completely changing the electronic properties of devices. completely changing the electronic properties of devices. Apart from 2D crystalline mono and few-layers devices, some research groups have studied Apart from 2D crystalline mono and few-layers devices, some research groups have studied different morphologies of MoS2, which may exhibit different sensing properties compared to both its different morphologies of MoS2, which may exhibit different sensing properties compared to both its bulk and 2D counterparts. Liu et al. [69] proposed a MoS2/Si pn junction devices for ammonia sensing, fabricated by magnetron sputtering from a MoS2 target, with a peculiar vertical structure Materials 2017, 10, 1418 10 of 22 instead of the more common planar one. This device was able to sense high concentration of ammonia, although with a low response (∆G/G ≈19.1%@200ppm NH3). At the same time, authors investigated the hydrogen sensing performances of this MoS2/Si device [70]. In presence of 30% relative humidity in air, proposed device was able to detect 5000 ppm of H2 with a response (∆I/I) of 15.4%. Moreover, controlling the relative humidity during measurements, authors asserted that water molecules have no effect on electrical properties of the material, but they compete with hydrogen molecules occupying the same surface sites. Yan et al. [71] mixed ZnO nanoparticles with MoS2 nanosheets grown by hydrothermal methods, and evaluated the gas sensing performances of conductometric devices toward some VOCs including ethanol. Optimal working temperature of pure and ZnO-coated devices were detected at 240 ◦C and ◦ 260 C, respectively, resulting in a response (Rair/Rgas) of the latter equal to 42.8@50ppm of ethanol. Response to other VOCs such as methanol was significantly lower, making the devices partially selective to ethanol. Sponge-like structures of MoS2 were prepared by Yu et al. [72] by hydrothermal technique and integrated into a conductometric device. They identified 150 ◦C as optimal sensing temperature for NO2 detection, and measured a maximum response (Rgas/Rair) of 78% to 50 ppm of NO2, diluted in air. The material behaves like a p-type semiconductor, and, even though the recovery of the baseline was thermally assisted in vacuum (650 ◦C for 1 h), reported response was extremely stable showing a maximum difference of ≈1% during one week. Another porous structure was proposed by Dwivedi et al. [73]. p-type silicon wafer was etched by electrochemical anodization to obtain a porous silicon substrate, on which metallic molybdenum was deposited on top by magnetron sputtering. The film was then oxidized to obtain MoO3, which was converted to n-type MoS2 by sulfurous film conversion as described in [41], forming a p-n junction. Porous MoS2 samples were characterized for the detection of methanol, ethanol, acetone and other VOCs, using nitrogen as carrier gas. Sensor response ∆R/R was quite low at 1 ppm, but porous samples performed over five times better than flat MoS2. The enhancement may be attributed to increased surface area, and to the barrier effect of p-p junction between porous and flat silicon. Moreover, the response was stable over more than two months. Quantum dots (QDs) of MoS2 and graphene oxide (GO) were mixed together to create a hybrid sensing material by Yue et al. [74]. GO and MoS2 powders were processed to obtain QDs liquid solution with G/M mass ratios of 1:1, 3:1 and 5:1, and then characterized to confirm the morphological, optical and gas sensing properties. Hybrid G/M QDs performed better than their counterparts alone did in detecting both NO2 and NH3. In particular, 3:1 device showed the highest response (∆R/R) thanks to charge transfer mechanism from adsorbed molecules and p-type QDs. All samples, however, exhibited a drift during recovery in N2. Interestingly, illumination of samples with 532 nm light source did not influenced significantly the performances of devices, and was due to local heating. For monolayer MoS2, the photo-thermoelectric effect is more dominant to the photocurrent than photoexcited electron–hole pairs across the Schottky barriers [75]. Electrohydrodynamic (EHD) printing process was realized by Lim et al. [76] to deposit a uniform distribution of exfoliated MoS2 flakes on desired substrates, to prepare conductometric chemical sensors. Yan et al. [77] demonstrated the ability of MoS2 nanosheets in preventing the aggregation of dispersed SnO2 nanoparticles. The presence of MoS2 allowed decreasing the optimal working ◦ ◦ temperature from 340 C to 280 C, exhibiting a response Rgas/Rair of ≈50 to 50 ppm of ethanol. However, Cui et al. [78] showed that decorating SnO2 nanocrystals could stabilize MoS2 nanosheets in air. Quite surprisingly, the combination of n-MoS2 and n-SnO2 fabricated by wet chemistry resulted in a p-type behavior of the heterostructure, and was able to dramatically enhance the stability, reproducibility and sensibility of the response and devices, in particular for NO2 detection. A similar switch in the semiconducting behavior of MoS2-based heterojunctions was detected by Zhao et al. [79] in well-aligned MoS2-decorated TiO2 nanotubes. Finally, Zhou et al. [80] reported in 2017 the synthesis Materials 2017, 10, 1418 11 of 22

of hybrid rGO/MoS2 composites via wet chemistry, and a comparison of NO2 sensing performances of pure rGO and rGO/MoS2. Baseline resistance of the latter was one order of magnitude higher than rGO device, and the response of the sensor was doubled. The effect of temperature, relative humidity and stoichiometry of the material on the sensing properties, including stability of the devices, was also discussed. Table2 reports a summary of chemical sensing MoS 2 devices found in literature.

Table 2. Summary of MoS2 gas sensing devices, target chemical compounds and performances. 1 2 3 RT stands for room temperature ( in N2 atmosphere; in Ar atmosphere; in presence of a relative humidity (RH) of 30%; 4 in presence of a relative humidity (RH) of 45%).

Gas and Ref. Material Growth Technique Device Type Performances Temperature

NH3-RT <2%@20ppm [58] MoS2 CVD Resistive NO2-RT ≈27%@20ppm 1 [59] MoS2 CVD Resistive NO2-RT ≈120%@1ppm 1 NH3-RT ≈50%@200ppm [61] MoS2 Mechanical Exfoliation FET 1 NO2-RT ≈400%@20ppm 2 NH3-RT ≈50%@200ppm [65] MoS2 CVD FET 2 NO2-RT ≈50%@100ppb ◦ [66] MoS2 Liquid Exfoliation Resistive NO2-200 C 5.8@1ppm

[68] MoS2 + Pd NPs Drop Casting + Evaporation Resistive H2-RT ≈35%@1%

NH3-RT ≈300%@50ppm [69,70] MoS2/Si Magnetron Sputtering Resistive pn junction 3 H2-RT 15.4%@5000ppm ◦ [71] MoS2 + ZnO Hydrothermal Resistive Ethanol-260 C 42.8@50ppm ◦ [72] MoS2 Hydrothermal Resistive NO2-150 C 78%@50ppm 1 [73] MoS2 Porous Sputtering + Film Conversion Resistive Ethanol-RT ≈2%@1ppm 1 NO2-RT ≈35%@10ppm [74] MoS2/GO QDs Exfoliation + Sonication Resistive 1 NH3-RT ≈20%@10ppm ◦ 1 NO2-100 C ≈10%@10ppm [76] MoS2 Flakes Sonication Resistive ◦ 1 CNH3-100 C ≈35%@10ppm ◦ [77] SnO2@MoS2 Hydrothermal Resistive Ethanol-280 C ≈50@50ppm

[78] SnO2@MoS2 Hydrothermal FET NO2-RT ≈28%@10ppm ◦ 4 [79] MoS2@TiO2 Hydrothermal Resistive Ethanol-150 C 14.2@100ppm

[80] MoS2/rGO Hydrothermal Resistive NO2-RT ≈60%@2ppm 1 [81] MoS2 Liquid Exfoliation FET NO2-RT ≈11%@1ppm

4. Molybdenum Diselenide (MoSe2) Chemical Sensors

Atomically thin MoSe2 is another good candidate for the fabrication of nanosized electronic devices [82]. It has a low bandgap (1.1–1.5 eV) and an appreciable mobility (≈50 cm2 V−1s−1), which allow the fabrication of switchable transistors and sensitive photodetectors devices. Thanks to its optical properties, it also proved to be a good candidate for the fabrication of fiber-optic humidity detectors [83]. Chemical vapor deposition (CVD), briefly illustrated in Figure 10, is the most common technique to prepare very thin flakes directly on target substrates [84–86], for the fabrication of functional devices. Molecular bean epitaxy (MBE) was also explored [87]. While MoSe2 electronic and optical performances have been largely investigated, few reports exist on its chemical sensing behavior. The first reported MoSe2 gas sensing device was presented by Late et al. in 2014 [88]. They mechanically exfoliated a flake of MoSe2 from a bulk crystal, which was deposited on SiO2/Si substrate. Electrodes were deposited by electron beam lithography (EBL). Although authors fabricated a field effect transistor, it was used as a simple conductometric device, in which gate bias was put to zero. Sample was exposed to different concentration of ammonia (50–500 ppm) at room temperature, followed by a recovery in Ar gas to avoid oxygen doping of the sensing material. Response, defined as ∆R/R, was extremely high, exceeding 1000 in presence of 500 ppm of NH3 (Figure 11c). However, Materials 2017, 10, 1418 12 of 22

Materials 2017, 10, 1418 12 of 22

it reduced quickly as the gas concentration decreased, and it showed an irregular dynamic behavior at low concentrations (Figure 11b). Authors detected fast response and slightly longer recovery time, due to the strong adsorption of ammonia molecules that are difﬁcult to remove from the surface of the Materialssensing 2017, 10 material., 1418 12 of 22

Figure 10. (a) Schematic of CVD process between the seed and target substrates, where selenium reacts with MoO3 nanosheets to form MoSe2 layers on the top substrate; (b) schematic of furnace setup; and (c) typical optical images of monolayer triangles, with some small bi-layered domains in darker color. (a,b) Reprinted with permission from [85]. Copyright (2015) Wiley-VCH Verlag GmbH. (c) Reprinted with permission from [86]. Copyright (2014) American Chemical Society.

The first reported MoSe2 gas sensing device was presented by Late et al. in 2014 [88]. They mechanically exfoliated a flake of MoSe2 from a bulk crystal, which was deposited on SiO2/Si substrate. Electrodes were deposited by electron beam lithography (EBL). Although authors fabricated a field effect transistor, it was used as a simple conductometric device, in which gate bias was put to zero. Sample was exposed to different concentration of ammonia (50–500 ppm) at room temperature, followed by a recovery in Ar gas to avoid oxygen doping of the sensing material. Response, defined as ∆R/R, was extremely high, exceeding 1000 in presence of 500 ppm of NH3 (Figure 11c). However, it reduced quickly as the gas concentration decreased, and it showed an irregular dynamic behavior at low concentrations (Figure 11b). Authors detected fast response and slightly longerFigure recovery11. (a) Optical time, image due gas to of the MoSe strong2 sensor adsorption device; (b) NH of3 sensingammonia response molecules as function that of aregas difficult Figure 11. (a) Optical image gas of MoSe2 sensor device; (b) NH3 sensing response as function of gas to removeconcentration;concentration; from the surface andand ((cc)) linearoflinear the plotplot sensing ofof sensitivitysensitivity material. ofof MoSe2MoSe2 gasgas sensorsensor device as a function of NH3 gas concentrationconcentration (ppm).(ppm). Reprinted with with permission permission fr fromom [88]. [88 ].Copyright Copyright (2014) (2014) AIP AIP Publishing Publishing LCC. LCC: Melville, NY, USA. Baek et al. characterized CVD grown MoSe2 flakes towards NO2, in both diode and FET configuration [89], transferred on SiO2/Si substrate by mechanical exfoliation (Figure 12). In FET Baek et al. characterized CVD grown MoSe2 flakes towards NO2, in both diode and FET configuration, this sensor exhibited high response (∆I/I) resulting in more than 1900 at very high NO2 configuration [89], transferred on SiO2/Si substrate by mechanical exfoliation (Figure 12). In FET concentration of 300 ppm, using N2 as carrier and recovery gas. This amount of NO2 by far exceeded configuration, this sensor exhibited high response (∆I/I) resulting in more than 1900 at very high any regulated threshold (EU regulation: <0.1 ppm for 1-h exposure [90]): it is higher than the IDLH NO concentration of 300 ppm, using N as carrier and recovery gas. This amount of NO by far value2 (20 ppm), potentially being lethal 2to living beings [91]. High sensitivity of this gas sensor2 is exceeded any regulated threshold (EU regulation: <0.1 ppm for 1-h exposure [90]): it is higher than the attributed to changes in the gap states near the valence band induced by the NO2 gas absorbed on the IDLH value (20 ppm), potentially being lethal to living beings [91]. High sensitivity of this gas sensor MoSe2. Moreover, proposed device exhibited fast response times and rapid on-off switching. Authors is attributed to changes in the gap states near the valence band induced by the NO gas absorbed also performed quantum transport simulations to verify the I-V characteristics, which2 resulted in a on the MoSe . Moreover, proposed device exhibited fast response times and rapid on-off switching. very good agreement2 with experimental data. AuthorsFigure 11. also (a) performed Optical image quantum gas of transport MoSe2 sensor simulations device; to (b verify) NH3 the sensing I-V characteristics, response as function which resulted of gas in a very good agreement with experimental data. concentration; and (c) linear plot of sensitivity of MoSe2 gas sensor device as a function of NH3 gas concentrationAlthough (ppm). functionalization Reprinted with of TMDs permission materials from is [88]. quite Copyright challenging (2014) due AIP to their Publishing intrinsic LCC. nature, Choi et al. were successful in doping MoSe2 with Nb atoms [92]. They fabricated Nb-doped MoSe2

Baek et al. characterized CVD grown MoSe2 flakes towards NO2, in both diode and FET configuration [89], transferred on SiO2/Si substrate by mechanical exfoliation (Figure 12). In FET configuration, this sensor exhibited high response (∆I/I) resulting in more than 1900 at very high NO2 concentration of 300 ppm, using N2 as carrier and recovery gas. This amount of NO2 by far exceeded any regulated threshold (EU regulation: <0.1 ppm for 1-h exposure [90]): it is higher than the IDLH value (20 ppm), potentially being lethal to living beings [91]. High sensitivity of this gas sensor is attributed to changes in the gap states near the valence band induced by the NO2 gas absorbed on the MoSe2. Moreover, proposed device exhibited fast response times and rapid on-off switching. Authors also performed quantum transport simulations to verify the I-V characteristics, which resulted in a very good agreement with experimental data.

MaterialsMaterials2017 2017, ,10 10,, 1418 1418 1313 of of 22 22

conductometric devices on sapphire substrates by a combination of different techniques for NO2 gas sensing. Firstly, MoO3 thin film (3 nm) was deposited by thermal evaporation, followed by one or five plasma enhanced atomic layer deposition cycles of Nb2O5 (PEALD) on top of MoO3 layer, with a thickness of 0.8 nm and 1.6 nm, respectively. Thermal selenization was carried out by film conversion technique similar to the one described in [41]. Niobium doping resulted in a significant decrease of electrical resistance, up to four order of magnitude, due to the metallic characteristics of NbSe2 [93]: five-cycle Nb samples experienced a doping-induced semiconductor-metal transition. ◦ Different from previous works, measurements were carried out at 150 C in both N2 and air, confirming the p-typeFigure behavior 12. (a) Optical of the image; material. (b) Atomic Slightly Force doped Microscopy MoSe2 (AFM)was able image; to achieve (c) XRD thepattern; highest (e) Fast response

(∆R/R),Fourier however Transform all devices(FFT); and suffered (f) inverse a significantFFT image of drift CVD-grown of the baselinesmultilayer MoSe (Figure2 flake. 13a,b). The thickness Interestingly, the authorsof the MoSe took2 flake in consideration is about 41 nm. the (d) stabilitySensitivity of vs. the NO device2 concentration, over time. where After the dashed a period blue of line 4 months, is Nb-dopedbased deviceson a model, exhibited and the small red square electrical symbols resistance represent changes. the data On from the the contrary, experiments. pure Reprinted MoSe2 devices sufferedwith from permission a strong from oxidation [89]. Copyright that resulted 2017 Springer-Verlag in a 60% change Berlin of resistanceHeidelberg. (Figure 13c). The response to NO2 was affected similarly (Figure 13d), confirming the enhancement of Nb-doping not only on the sensitivityAlthough but also functionalization on the stability of ofTMDs the devices. materials is quite challenging due to their intrinsic nature, Materials 2017, 10, 1418 13 of 22 Choi et al. were successful in doping MoSe2 with Nb atoms [92]. They fabricated Nb-doped MoSe2 conductometric devices on sapphire substrates by a combination of different techniques for NO2 gas sensing. Firstly, MoO3 thin film (3 nm) was deposited by thermal evaporation, followed by one or five plasma enhanced atomic layer deposition cycles of Nb2O5 (PEALD) on top of MoO3 layer, with a thickness of 0.8 nm and 1.6 nm, respectively. Thermal selenization was carried out by film conversion technique similar to the one described in [41]. Niobium doping resulted in a significant decrease of electrical resistance, up to four order of magnitude, due to the metallic characteristics of NbSe2 [93]: five-cycle Nb samples experienced a doping-induced semiconductor-metal transition. Different from previous works, measurements were carried out at 150 °C in both N2 and air, confirming the p-type behavior of the material. Slightly doped MoSe2 was able to achieve the highest response (∆R/R), however all devices suffered a significant drift of the baselines (Figure 13a,b). Interestingly, the Figure 12. (a) Optical image; (b) Atomic Force Microscopy (AFM) image; (c) XRD pattern; (e) Fast authorsFigure took 12. (ina) Opticalconsideration image; (b )the Atomic stability Force of Microscopy the device (AFM) over image; time. (c )After XRD pattern; a period (e) Fastof Fourier4 months, Fourier Transform (FFT); and (f) inverse FFT image of CVD-grown multilayer MoSe2 flake. The thickness Nb-dopedTransform devices (FFT); exhibited and (f) inverse small FFTelectrical image re ofsistance CVD-grown changes. multilayer On the MoSe contrary,2 flake. pure The thicknessMoSe2 devices of of the MoSe2 flake is about 41 nm. (d) Sensitivity vs. NO2 concentration, where the dashed blue line is sufferedthe MoSe from2 aflake strong is about oxidation 41 nm. that (d) resulted Sensitivity in vs.a 60% NO change2 concentration, of resistance where (Figure the dashed 13c). blue The lineresponse is basedbased onon aa model,model, andand thethe redred squaresquare symbolssymbols representrepresent thethe datadata fromfrom thethe experiments. experiments. ReprintedReprinted to NO2 was affected similarly (Figure 13d), confirming the enhancement of Nb-doping not only on with permission from [89]. Copyright 2017 Springer-Verlag Berlin Heidelberg. the sensitivitywith permission but also from on [89 the]. Copyright stability of 2017 the Springer-Verlag devices. Berlin Heidelberg. Although functionalization of TMDs materials is quite challenging due to their intrinsic nature, Choi et al. were successful in doping MoSe2 with Nb atoms [92]. They fabricated Nb-doped MoSe2 conductometric devices on sapphire substrates by a combination of different techniques for NO2 gas sensing. Firstly, MoO3 thin film (3 nm) was deposited by thermal evaporation, followed by one or five plasma enhanced atomic layer deposition cycles of Nb2O5 (PEALD) on top of MoO3 layer, with a thickness of 0.8 nm and 1.6 nm, respectively. Thermal selenization was carried out by film conversion technique similar to the one described in [41]. Niobium doping resulted in a significant decrease of electricalFigure resistance, 13. (a) Dynamic up to response four order of the of Nb-dopedmagnitude, MoSe due2 devices to the at metallic NO2 concentrations characteristics ranging of NbSefrom 2 [93]: Figure 13. (a) Dynamic response of the Nb-doped MoSe2 devices at NO2 concentrations ranging five-cycle3 to 50 Nb ppm; samples and ( bexperienced) responses of a thedoping-induced three devices as semiconductor-metal a function of the NO2 gastransition. concentration. Different (c) from from 3 to 50 ppm; and (b) responses of the three devices as a function of the NO2 gas concentration. previousResistance works, change measurements data over 120 were days: carried Nb-doped out MoSe at 1502 devices °C in showedboth N 2negligible and air, resistanceconfirming change, the p -type (c) Resistance change data over 120 days: Nb-doped MoSe2 devices showed negligible resistance change, behaviordifferently of the from material. MoSe2 devices.Slightly ( ddoped) Comparison MoSe2 ofwas the ablelong-term to achieve stability the of thehighest three devicesresponse with (∆ R/R), differently from MoSe2 devices; (d) Comparison of the long-term stability of the three devices with howeverrespectrespect all to to devices the the gas gas sensing sensingsuffered response. response. a significant All All gas-sensing gas-sensing drift testsof tests the were were baselines performed performed (Figure at at an an operating operating 13a,b). temperature Interestingly,temperature of the authors150of 150 ◦C.took Reprinted°C. inReprinted consideration with with permission permission the fromstability from [92]. [92] Copyrightof. Copyrightthe device (2017) (2017) Americanover American time. Chemical After Chemical Society:a period Society. Washington, of 4 months, Nb-dopedDC, USA. devices exhibited small electrical resistance changes. On the contrary, pure MoSe2 devices sufferedTable from 3 reports a strong a summaryoxidation ofthat chemical resulted sensing in a 60% MoSe change2 devices of resistance found in (Figure literature. 13c). The response to NO2 was affected similarly (Figure 13d), confirming the enhancement of Nb-doping not only on Table3 reports a summary of chemical sensing MoSe 2 devices found in literature. the sensitivity but also on the stability of the devices.

Figure 13. (a) Dynamic response of the Nb-doped MoSe2 devices at NO2 concentrations ranging from 3 to 50 ppm; and (b) responses of the three devices as a function of the NO2 gas concentration. (c)

Resistance change data over 120 days: Nb-doped MoSe2 devices showed negligible resistance change, differently from MoSe2 devices. (d) Comparison of the long-term stability of the three devices with respect to the gas sensing response. All gas-sensing tests were performed at an operating temperature of 150 °C. Reprinted with permission from [92]. Copyright (2017) American Chemical Society.

Table 3 reports a summary of chemical sensing MoSe2 devices found in literature.

Materials 2017, 10, 1418 14 of 22

Table 3. Summary of MoSe2 gas sensing devices, target chemical compounds and performances. 1 2 RT stands for room temperature ( in Ar atmosphere; in N2 atmosphere).

Ref. Material Growth Technique Device Type Gas and Temperature Performances 1 [88] MoSe2 Mechanical Exfoliation Resistive/FET NH3-RT ≈200@200ppm 2 [89] MoSe2 CVD + Mechanical Exfoliation Resistive/FET NO2-RT 1907@300ppm ◦ 2 [92] Nb-doped MoSe2 ALD + Film Conversion Resistive NO2-150 C 8%@3ppm

5. Molybdenum Ditellurite (MoTe2) Chemical Sensors

Molybdenum ditellurite (MoTe2) has a very small bandgap of only 0.8–1.1 eV depending on the morphology, lower than many TMDs including other molybdenum dichalcogenides [94]. It has a strong absorption throughout the entire solar spectrum, and thus it has been adopted as electrode material in photovoltaic cells [95]. Thanks to its good room temperature carrier mobility (up to 200 cm2 V−1 s−1)[96], and to the possibility to tune the growth to obtain n- and p-type semiconductors, it is well suited for the fabrication of transistors and LED [97–99]. Moreover, it is possible to force a phase transition from semiconducting α-MoTe2 to metallic β-MoTe2, leading to the formation of interesting nanosized Schottky junctions [19,100] and showing a unique potential in phase-transition devices [101]. Finally, it has a low thermal conductivity [102] and may show a superconductive behavior [103,104]. However, as reported in the Introduction, ditellurite materials are the least common class of TMDCs, and very few reports exist on the application of MoTe2 as chemical sensors. This material is usually synthetized by chemical vapor transport [99], exfoliation [17], CVD [105,106] or MBE [107], and then transferred on destination substrates used for the preparation of functional devices. The most common configuration is the field effect transistor, in which a MoTe2 flake is deposited on source and drain electrodes on a passivated silicon substrate. Unfortunately, very thin layers of MoTe2 are not stable in air, and readily oxidize to MoO3 and TeO2 keeping the two dimensional morphology [107,108]. However, oxidation process is not only due to the presence of oxygen itself. The introduction of pure oxygen in a low vacuum environment, even at high temperature, is not enough to oxidize the material: possibly water or other atmospheric oxidant are involved too. This from one side confirms the sensibility of the material toward the surrounding atmosphere, but from the other affects the stability of MoTe2 chemical sensor devices, which potentially have a limited lifetime and may exhibit a gradual loss of performance. Lin et al. [109] were the first in 2015 investigating the possible application of MoTe2 in field effect transistors for environmental sensing. They synthetized semiconducting α-MoTe2 by mechanical exfoliation of a bulk crystal grown by chemical vapor transport, which was deposited on SiO2/Si [99]. Bulk silicon was connected as gate contact, while two thin Ti/Au contacts acted as source and drain electrodes. Exposition of the devices to ambient air resulted in a reduction of carrier mobility by half compared to the original values in vacuum, and in an increased hysteresis of the transfer characteristics due to the adsorption of oxygen molecules. Quite interestingly, authors demonstrate that low frequency electronic noise of these devices depends on MoTe2 conductive channel itself rather than on contact barriers, and that this noise is affected strongly by the interaction with surrounding atmosphere. However, no performance evaluations are reported. A functional FET device based on MoTe2 was presented, instead, by Feng et al. in 2016 [110], starting from an exfoliated flake deposited on passivated silicon substrate, similarly to [107]. Authors could tune the semiconducting behavior of the material from p- to n-type by thermal annealing, and evaluated the performances of these p- and n-type devices toward NO2 and NH3, respectively, some typical air pollutant. Both devices were highly dependent on gate bias voltage, with zero bias as optimal condition for sensing measurement (sub-threshold region) [111]. In these conditions, p-MoTe2 exhibited a response (defined as ∆G/G%) of 140% toward 100 ppb of NO2, exceeding 1000% toward 1 ppm, at room temperature. In the same experimental condition n-MoTe2, instead, resulted in a response of ≈30% toward 2 ppm of NH3. Response and recovery time at zero gate bias were extremely fast, Materials 2017, 10, 1418 15 of 22 Materials 2017, 10, 1418 15 of 22

MaterialsrecoveringThe 2017same, 10 the ,authors 1418 baseline achieved in less than a 10further min for improvement both devices and to gases.NH3 detection On the contrary, by illuminating the application15 nof-type 22 MoTeof2 aFET positive devices or negative with a continuous bias always resultedlight source in lower [112 performances.]. Optical, atomic force microscopy (AFM) and The same authors achieved a further improvement to NH3 detection by illuminating n-type transmissionThe sameelectron authors microscopy achieved a(TEM) further pictures improvement are reported to NH3 detectionin Figure by 14a–c, illuminating respectively.n-type The MoTe2 FET devices with a continuous light source [112]. Optical, atomic force microscopy (AFM) and effectMoTe of both2 FET photon devices energy with a (wavelength) continuous light and source intensity [112 ].was Optical, investigated, atomic force always microscopy resulting (AFM) in higher transmission electron microscopy (TEM) pictures are reported in Figure 14a–c, respectively. The performancesand transmission of illuminated electron microscopydevices compared (TEM) pictures to the areone reported in dark. in As Figure demonstrated 14a–c, respectively. by previous effect of both photon energy (wavelength) and intensity was investigated, always resulting in higher investigations,The effect of zero both gate photon bias energy was applied (wavelength) during and electrical intensity measurements. was investigated, always resulting in performanceshigher performances of illuminated of illuminated devices devices compared compared to the tothe one one in in dark. dark. As As demonstrated by by previous previous investigations,investigations, zero zero gate gate bias bias was was applied applied during electricalelectrical measurements. measurements.

Figure 14. (a) Optical microscope image of the MoTe2 field-effect transistor (FET). Scale bar is 5 μm.

(bFigure) AtomicFigure 14.14. force (a()a Optical) Opticalmicroscopy microscope microscope (AFM image image) topography of of the MoTe image22 ﬁeld-effectfield-effect of the MoTetransistor transistor2 FET. (FET). (FET).Thickness Scale Scale barof bar the is is 5 MoTe µ5m; μm.2 is 3.4( bnm.()b Atomic) AtomicScale force bar force ismicroscopy microscopy5 μm. (c) (AFMHigh-resolution (AFM)) topography topography transmission image image of of the the elecMoTe MoTetron2 FET.FET. microscopy Thickness (TEM) of thethe MoTeimageMoTe22 isof a typical3.4is nm. 3.4 exfoliated nm.Scale Scale bar MoTe baris 5 is2μ film. 5m.µ m;(c )Scale (High-resolutionc) High-resolution bar is 2 nm. transmissionReprinted transmission fr omelec electron [112],tron microscopy microscopyMDPI, Basel, (TEM) Switzerland. imageimage ofof a typicala typical exfoliated exfoliated MoTe MoTe2 film.2 ﬁlm. Scale Scale bar bar is is2 nm. 2 nm. Reprinted Reprinted fr fromom [112], [112], MDPI, MDPI, Basel, Switzerland.Switzerland.

The response of the MoTe2 sensor toward NH3 is enhanced remarkably under light illumination The response of the MoTe2 sensor toward NH3 is enhanced remarkably under light illumination from near-infraredThe response (900 of the nm) MoTe to UV2 sensor region toward (254 NH nm3 ),is keeping enhanced the remarkably same light under intensity light illumination (2.5 mW/cm2), from near-infrared (900 nm) to UV region (254 nm), keeping the same light intensity (2.5 mW/cm2 2), as reportedfrom near-infrared in Figure 15a,b. (900 nm) Even to UV though region MoTe (254 nm),2 has keeping a small the bandgap same light compat intensityible (2.5with mW/cm near-infrared), as reported in Figure 15a,b. Even though MoTe2 has a small bandgap compatible with near-infrared energy,as reported 254 nm in UV Figure light 15a,b. is Eventhe most though effective MoTe2 hasin aenhancing small bandgap the compatibleresponse of with the near-infrared FET device. In energy,energy, 254 254 nm UVUV light light is theis the most most effective effective in enhancing in enhancing the response the response of the FET of device. the FET In particular, device. In particular, it ranges from 4% (300 ppb) to 25% (30 ppm) under dark condition, whereas it range from particular,it ranges it from ranges 4% (300from ppb) 4% (300 to 25% ppb) (30 to ppm) 25% under(30 ppm) dark under condition, dark condition, whereas it whereas range from it range 100% from to 100% to 790% under UV light, 50 times higher than dark response. UV illumination has the side effect 100%790% to under790% under UV light, UV 50light, times 50 times higher higher than darkthan response.dark response. UV illumination UV illumination has the has side the effectside effect of of forcing a behavior change from p- to n-type, due to the removal of residual O2 molecules from of forcingforcing a a behavior behavior change change from fromp- to pn- -type,to n-type, due todue the to removal the removal of residual of residual O2 molecules O2 molecules from MoTe from2 MoTe2 flake. Oxygen removal is deemed the reason of enhanced response of the devices, releasing MoTeﬂake.2 flake. Oxygen Oxygen removal removal is deemed is deemed the reason the reason of enhanced of enhanced response response of the devices, of the devices, releasing releasing more 2 moremoresurface surface surface active active active sites. sites. UVsites. light UV UV intensity lightlight intensityintensity has a similar has effect:a similarsimilar applying effect:effect: an applying intensityapplying froman an intensity 0.25intensity mW/cm from from to0.25 0.25 2 2 2 mW/cm2.5 mW/cm to2 2.5 mW/cmthe response 2the response to 30 ppm to of 30 NH ppmincreases of NH from3 increases 290% to from 790% 290% (Figure to 15 790%c,d). (Figure 15c,d). mW/cm to 2.5 mW/cm the response to 30 ppm3 of NH3 increases from 290% to 790% (Figure 15c,d).

Figure 15. Cont.

Materials 2017, 10, 1418 16 of 22 Materials 2017, 10, 1418 16 of 22

Figure 15. (a) Real-time conductance change of the MoTe2 sensor upon exposure to different Figure 15. (a) Real-time conductance change of the MoTe2 sensor upon exposure to different concentrations of NH3 in the dark and under light illumination; (b) calculated response and calibration concentrations of NH3 in the dark and under light illumination; (b) calculated response and curves of the MoTe2 sensor as a function of wavelength (photon energy); (c) real-time conductance calibration curves of the MoTe2 sensor as a function of wavelength (photon energy); (c) real-time change of the MoTe2 sensor upon exposure under 254 nm UV light illumination; and (d) calculated conductance change of the MoTe2 sensor upon exposure under 254 nm UV light illumination; and (d) response and calibration curves of the MoTe2 sensor as a function of UV light intensity. Reprinted calculatedfrom [112 response], MDPI, and Basel, calibration Switzerland. curves of the MoTe2 sensor as a function of UV light intensity. Reprinted from [112], MDPI, Basel, Switzerland.

Table4 reports a summary of chemical sensing MoTe 2 devices found in literature. Table 4 reports a summary of chemical sensing MoTe2 devices found in literature. Table 4. Summary of MoTe2 gas sensing devices, target chemical compounds and performances. RT stands for room temperature. * in N2 atmosphere. Table 4. Summary of MoTe2 gas sensing devices, target chemical compounds and performances.

RT stands for room temperature. * in N2 atmosphere. Ref. Material Growth Technique Device Type Gas and Temperature Performances

Ref. [109 Material] MoTe 2 GrowthMechanical Technique Exfoliation Device FET Type Gas and Air Temperature Performances -

[109] MoTe2 Mechanical Exfoliation FET NO2-RT Air 140%@100ppb - * [110] MoTe2 Exfoliation FET NHNO3-RT2-RT ≈30%@2ppm140%@100ppb * * [110] MoTe2 Exfoliation FET [112] MoTe2 Mechanical Exfoliation FET + Light Illumination NHNH3-RT3-RT 100%@300ppb≈30%@2ppm * * [112] MoTe2 Mechanical Exfoliation FET + Light Illumination NH3-RT 100%@300ppb * 6. Conclusions 6. Conclusions The properties of bulk TMDs, especially molybdenum dichalcogenides, are diverse, and they get evenThe moreproperties interesting of bulk at nanoscale TMDs, levelespecially in form molybdenum of 2D thin layers. dichalcoge They sharenides, the same are facilediverse, synthesis and they get eventechniques, more mainlyinteresting based at on nanoscale exfoliation oflevel bulk in material, form of which 2D thin are reminiscentlayers. They of grapheneshare the and same other facile synthesis2D materials. techniques, However, mainly the based chemistry on exfoliation of MoX2 compounds of bulk material, offers which opportunities are reminiscent for going of beyond graphene graphene and opening up new fundamental and technological pathways for inorganic 2D materials. and other 2D materials. However, the chemistry of MoX2 compounds offers opportunities for going Metal oxide materials are still the state-of-the-art for the fabrication of environmental chemical beyond graphene and opening up new fundamental and technological pathways for inorganic 2D sensors, especially in form of nanostructures such as nanowires, nanoparticles and nanotubes, thanks materials.to their superior performances that are the results of years of research activities. However, they have someMetal limitations oxide materials which are are pushing still the the state-of-the-art research to look forfor alternative the fabrication sensing of materials. environmental Amongthese, chemical sensors,emerging especially TMDs in play form an importantof nanostructures role, but are such not theas nanowires, only protagonists nanoparticles [113]. Black and Phosphorous nanotubes, (BP), thanks to theirfor instance,superior has performances proven to be quitethat are sensitive the results and selective of years to of NO research2 gas [114 activities.]. MoS2, instead, However, is sensitive they have someto limitations many other which different are chemical pushing compounds the research also [to115 look], and for thus alternative is not favorable sensing for materials. the fabrication Among these,of highlyemerging selective TMDs devices, play whichan important is also one role, of the but strongest are not limits the of only metal protagonists oxides. Graphene [113]. has Black Phosphorousmuch lower (BP), performances for instance, for has NO 2provenand in generalto be quite for chemicalsensitive sensing, and selective therefore to NO cannot2 gas be [114]. used ifMoS2, instead,sensitivity is sensitive is a requirement. to many other different chemical compounds also [115], and thus is not favorable All these extremely innovative materials, however, pose challenges when their integration into for the fabrication of highly selective devices, which is also one of the strongest limits of metal oxides. sensing devices is performed at industrial scale. Mechanical exfoliation is not feasible to produce large Graphene has much lower performances for NO2 and in general for chemical sensing, therefore amount of devices in a reproducible way, and liquid exfoliation has some limitations too. Chemical cannotvapor be depositionused if sensitivity or thin ﬁlm is conversiona requirement. are better suited, but the high temperatures required could be incompatibleAll these extremely with traditional innovative silicon ma processing.terials, however, pose challenges when their integration into sensing devices is performed at industrial scale. Mechanical exfoliation is not feasible to produce large amount of devices in a reproducible way, and liquid exfoliation has some limitations too. Chemical vapor deposition or thin film conversion are better suited, but the high temperatures required could be incompatible with traditional silicon processing. According to the very first reports, among molybdenum dichalcogenides, MoTe2 appears as the most exciting material for gas sensing applications, due to its extreme sensitivity to the surrounding atmosphere. Despite the very interesting results reported in literatures, in real field applications

Materials 2017, 10, 1418 17 of 22

According to the very first reports, among molybdenum dichalcogenides, MoTe2 appears as the most exciting material for gas sensing applications, due to its extreme sensitivity to the surrounding atmosphere. Despite the very interesting results reported in literatures, in real field applications things could be different from expected. In many works, for example, performances were evaluated using nitrogen or argon as carrier gas instead of synthetic air: due to the lack of oxygen, these sensing measurements are not truly representative of the performances of the devices, avoiding the degradation of the material due to progressive oxidative processes. Therefore, it is very difficult to have the complete picture of the performances of these materials in terms of sensibility and stability over time, in particular for MoSe2 and MoTe2 which are not investigated properly yet. Their spontaneous degradation in atmosphere is one of the biggest limiting factor for the use of these materials in commercial devices. Research groups who have dealt with stability issues stress that, in many cases, recovery of devices was not complete, even for MoS2 that is the most studied TMD material. In particular, in the case of environmental sensors, it is difficult to release NO2 and NH3 molecules from material. Thermally assisted recovery could be an option, but we lose one of the biggest advantage of these materials: the possibility to operate at room temperature. The combined use of light illumination, with tuned wavelength and intensity, proved to be a valid alternative to enhance the performances of the devices, but could induce modification in the structure of the materials or make them degrade faster. Another option is the chemical functionalization of the surface or even integration of other 2D materials, such as BP or graphene. This may also result in the stabilization of these MoX2 in presence of oxidizing atmosphere, and further enhance the selectivity among target chemical species. However, research activities on TMDs material are growing exponentially; therefore, it is reasonable to expect many scientific breakthrough in the next few years, which will be potentially able to revolutionize gas sensor market.

Acknowledgments: The author would like to thank all Sensor Laboratory staff members. Author Contributions: D.Z. conceived and wrote the paper. Conﬂicts of Interest: The author declares no conﬂict of interest.

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