Tin Diselenide (SnSe2) Van der Waals Semiconductor: Surface Chemical Reactivity, Ambient Stability, Chemical and Optical Sensors.

Tin diselenide (SnSe2) is a layered semiconductor with broad application capabilities in the fields of energy storage, photocatalysis, and photodetection. Here, we correlate the physicochemical properties of this van der Waals semiconductor to sensing applications for detecting chemical species (chemosensors) and millimeter waves (terahertz photodetectors) by combining experiments of high-resolution electron energy loss spectroscopy and X-ray photoelectron spectroscopy with density functional theory. The response of the pristine, defective, and oxidized SnSe2 surface towards H2, H2O, H2S, NH3, and NO2 analytes was investigated. Furthermore, the effects of the thickness were assessed for monolayer, bilayer, and bulk samples of SnSe2. The formation of a sub-nanometric SnO2 skin over the SnSe2 surface (self-assembled SnO2/SnSe2 heterostructure) corresponds to a strong adsorption of all analytes. The formation of non-covalent bonds between SnO2 and analytes corresponds to an increase of the magnitude of the transferred charge. The theoretical model nicely fits experimental data on gas response to analytes, validating the SnO2/SnSe2 heterostructure as a suitable playground for sensing of noxious gases, with sensitivities of 0.43, 2.13, 0.11, 1.06 [ppm]-1 for H2, H2S, NH3, and NO2, respectively. The corresponding limit of detection is 5 ppm, 10 ppb, 250 ppb, and 400 ppb for H2, H2S, NH3, and NO2, respectively. Furthermore, SnSe2-based sensors are also suitable for fast large-area imaging applications at room temperature for millimeter waves in the THz range.

1. Introduction

Following the advent of graphene [1,2,3,4], the scientific community has begun to consider layered semiconductors for their potential application to complement those of graphene [5,6,7,8,9], thus generating promising new technologies in various technological areas [10,11,12,13,14,15,16]. The class of materials of ‘van der Waals semiconductors’ is characterized by weak van der Waals bonds between the layers that allow their exfoliation by mechanical [17,18] and liquid-phase [19,20,21] exfoliation.

The most common van der Waals semiconductors have shown limitations, which compromise their technological development. For example, MoS2 and WS2 have a poor electric mobility of a few tens of cm2V−1s−1 at T = 300 K [22]; black phosphorus is instable in air and undergoes a rapid surface oxidation which degrades the morphology of the surface [23]; GaSe exhibits instability upon both laser and air exposure [24,25]; and PdSe2 [26,27] has a limited commercial potential, due to the constantly growing price of Pd (2000–2400 $/oz), nearly doubled in 2019–2021.

Tin diselenide (SnSe2) is a layered semiconductor constituted by Earth-abundant and cheap elements [28], which crystallizes in a layered CdI2–type structure with hexagonally packed layers of Sn atoms sandwiched between two layers of Se anions (Figure 1a,b) [29,30,31]. Differently from MoS2 and WS2, SnSe2 has a high intrinsic electron mobility at T = 300 K (462.6 cm2V−1s−1) and ultralow thermal conductivity (3.82 W m−1 K−1) [32]. Furthermore, SnSe2 exhibits pressure-induced periodic lattice distortion and, moreover its atomic structure can reversibly change from amorphous to crystalline upon laser heating, being a phase change memory material. Owing to these peculiarities, SnSe2 has high application capabilities in several fields, including superconductivity [33,34], Li− [29,35] and Na− [29,36] ion batteries, photodetection [37], photocatalysis [38,39], saturable absorbers for eye-safe lasers [40], and thermoelectricity [41,42].

Figure 1

(a) Side and (b) top views of the atomic structure of SnSe2. Green and blue balls denote Se and Sn atoms, respectively. Panel (c) reports the single-crystal XRD pattern from the (001) plane of SnSe2. The inset shows a photograph of an as-grown SnSe2 single crystal. Panel (d) reports the Raman spectrum of SnSe2 single crystal acquired at room temperature with a laser with wavelength λ = 632.8 nm.

Nevertheless, Sn-based chalcogenides suffer from rapid surface oxidation with the formation of surface tin-oxide phases [43,44]. Furthermore, during the crystal synthesis process, tin could already oxidize, modifying its overall transport properties [45]. Therefore, the use of Sn-based chalcogenides for technology transfer remains particularly arduous. Especially, stability in ambient atmosphere of SnSe2-based devices is related to the chemical reactivity of its surface.

Here, we unveil surface properties of SnSe2 single crystals by means of surface-science experiments and density functional theory (DFT). We demonstrate that the stoichiometric SnSe2 sample is chemically inert, while the presence of Se vacancies induces surface oxidation with the formation of a sub-nanometric SnO2 skin. We also explore the capability of SnSe2 to realize devices for sensors for detecting noxious gases and imaging applications with non-ionizing radiations. Especially, we show that chemical sensing is feasible only when the pristine SnSe2 surface is transformed into an heterostructure of SnO2/SnSe2. Concerning photodetection, we report the design of broadband SnSe2-based photodetectors interplayed by synergistic effects of multiple mechanisms. Considerably, the effect of hot electrons in ultrashort channel devices under strong light coupling results in outstanding performance in term of high responsivity at THz frequency.

2. Materials and Methods

Single crystals of SnSe2 were grown by Bridgman–Stockbarger (Figure 1c). Stoichiometric ratio of 1:2 was put on evacuated quartz ampoule. The growth was carried out in a vertical two-zone tube furnace. The obtained crystal was characterized with X-ray diffraction (XRD) on powders, as shown Figure 1c. From the XRD spectrum, we can conclude that the crystal structure is CdI2–type (space group P-3m1). The lattice parameters are a = 0.3804 nm and c = 0.6128 nm consistently with previous works [46,47,48,49]. We also carried out the XRD and Laue diffraction measurements on single crystals. Samples were exfoliated in situ by adhesive tape. The absence of contamination in grown single crystals is secured by the survey X-ray photoelectron spectroscopy (XPS) spectrum.

XPS experiments were carried out with synchrotron light at APE-HE beamline at the Elettra Synchrotron in Trieste, Italy.

High resolution electron energy loss spectroscopy (HREELS) experiments were performed with a Delta 0.5 spectrometer (Specs GmbH, Germany). Spectra were taken in specular geometry, with an impinging angle of 55° with respect to the perpendicular direction to the surface. The impinging energy is 3.5 eV.

Theoretical methods are reported in Section S1 of the Supplementary Materials.

Fabrication process and measurements of devices are reported in section S2 of the Supplementary Materials.

3. Results

3.1. Chemisorption of O2 and H2O on Bulk SnSe2

The Raman spectrum of the grown SnSe2 single crystal (Figure 1d) shows the Eg and A1g modes at 109 and 184 cm−1, respectively, congruently with previous reports [50,51,52]. The narrow (00l) diffraction peaks (Figure 1c) reveal the excellent crystallinity for our SnSe2 crystals.

To model surface chemical reactivity, the differential enthalpy ΔHads and the differential Gibbs free energy ΔG for the adsorption of water and oxygen at room temperature, as well as the decomposition energy ΔHdec for both gases were calculated. The possibility of different kinds of defects was energetically evaluated and the formation of one Se vacancy is particularly feasible (only 1.28 eV/Se). The influence of Se vacancies was explored from a single vacancy in the outermost surface layer (Figure 2a) up to larger concentration of Se vacancies. We also calculated the different possible positions for a second Se vacancy, finding that the most energetically favorable location (0.98 eV/Se) is to have the second vacancy in the next neighbor to the first vacancy. To model such a large number of vacancies, an outermost SnSe-like layer on the surface was also considered (Figure 2b). Calculations indicate that physisorption of molecular oxygen is feasible at all investigated surfaces, although it is more energetically favorable at Se vacancies (ΔG = −26.3 kJ/mol) rather than on defects-free SnSe2 (ΔG = −3.2 kJ/mol). The subsequent decomposition of molecular oxygen is an exothermic process for SnSe2, SnSe1.88, and SnSe. However, in the case of SnSe1.88 and SnSe the differential enthalpy of decomposition is much lower than the defect-free SnSe2 (−135.7, −236.1, and −42.3 kJ/mol, respectively). Thus, the oxidation rate should be greater on defective surfaces of SnSe2. After the decomposition of a single oxygen molecule, we assess the oxygenation of the whole surface, corresponding to an atomic structure with oxygen atom attached to each surface Se atom, and the successive oxidation in a metastable surface SnSe2O2 phase (see Figure 2c), whose lifetime is estimated to be < 1 ms and, consequently, its presence on the surface could be detected only with time-resolved experiments. Hence, first the Se atoms in the SnSe2O2 migrate to occupy the Se vacancies formed in the subsurface region and then the oxygen atoms from the SnSe2O2 oxidize the Sn atoms of the surface layer to form SnO2, see Figure 2d. The following chemical equation can describe this process as O with x < y, where SnSe2-y corresponds to Se defects in the substrate, which are partially or totally saturated by Se freed from top layer after formation of SnO2-skin.

Figure 2

Optimized atomic structure of (a) water molecules physisorbed at one Se-vacancy site; (b) decomposed oxygen molecule on SnSe surface layer; (c) metastable SnSe2O2 surface layer; and (d) SnO2-skin-terminated SnSe2.

We calculated the necessary energy to heal the Se vacancy by extracting Se from the SnO2 skin and the differential enthalpy for this reaction is −3.54 eV/Se for the oxidized surface of SnSe2 and −2.97 eV/Se for oxidized surface of SnSe1.88. These values cause a preferential oxidation of Sn with respect to Se and, subsequently, the surface oxide layer should be modelled as a SnO2/SnSe2 heterostructure.

Instead, the physisorption of water is energetically unfavorable on SnSe2. Although near the Se vacancy the energy barrier for water absorption decreases it always remains a metastable process with positive ΔG in the Se vacancy sites (ΔG = +3.4 kJ/mol). Even in the defect sites, a subsequent decomposition of water is extremely unfavorable (ΔHdec = +175.7 kJ/mol). This behavior can be understood because the water molecules interact with diselenides via the formation of non-covalent bonds between lone pairs of electrons on sp orbitals of oxygen and unoccupied orbitals of metal centers in substrate. In the case of SnSe2, some mismatch between the size of water molecule and lattice parameters of the substrate is not so favorable for the formation of the described non-covalent bonds. The contribution to the adsorption energy calculated from the energy cost of the substrate and molecule distortions decreases ΔH, making it lower than the TΔS contribution in the ΔG calculation. In the case of adsorption on an oxidized substrate, hydrogen bonds are established between water and substrate oxygen. The formation of these hydrogen bonds can occur at much broader range of positions of water on the substrate and, therefore, no contribution in ΔH comes from distortions of either substrate or water.

3.2. Experimental Validation of the Theoretical Model

Probing vibrational modes could afford additional information on surface chemical processes and, especially, physicochemical mechanisms ruling the formation of an oxide skin. In particular, high-resolution electron energy loss spectroscopy (HREELS) experiments on H2O-exposed tin selenides SnSex, with x ranging between 1 and 2 (SnSe, SnSe1.4, SnSe1.7, SnSe2), indicate the lack of chemisorbed molecules resulting from the presence of H2O, this is evident from the absence of O-H streching at 410–420 meV (molecular water) and 450 meV (hydroxyl groups) in the spectra in Figure 3 [53]. These findings are consistent with the positive Gibbs free energy of adsorption (corresponding to energetically unfavorable water adsorption) in Table 1. For a better comparison, we report in Figure 3 the vibrational data obtained by exposing other chalcogenides to the same dose of water (105 L). Unlike the surface of SnSex, a stable adsorption of water molecules was found on PtTe1.6 and, moreover, of hydroxyl groups on InSe. The absence of reactivity toward water of Sn-based chalcogenides makes them suitable for catalysis (in particular, photocatalytic water splitting [38], and hydrogen evolution reaction [54]) and drug delivery [55] (also bearing in mind that neither Sn nor Se are toxic).

Figure 3

Vibrational spectra around the energy of O-H intramolecular stretching, recorded upon dosing to 105 L of H2O at T = 300 K the surfaces of various tin-based selenides: SnSe2 (orange), SnSe1.7 (black), SnSe1.4 (green), and SnSe (blue). We also report vibrational acquired in the same conditions for water-dosed InSe (red) and PtTe1.6 (brown). The energy of the primary electron beam is 4 eV.

Table 1

Differential enthalpy ΔHads and differential Gibbs free energy ΔG for physisorption at room temperature and differential enthalpy of decomposition ΔHdec for molecular oxygen and water on the surface of bulk samples of SnSe2, SnSe1.88, and SnSe. For the decomposition of oxygen, we also report, in parenthesis, the differential enthalpy for the formation of an oxygenated surface and a SnO2-like layer.

Surface	Adsorbant	Physisorption		Decomposition
		ΔHads (kJ/mol)	ΔG (kJ/mol)	ΔHdec (kJ/mol)
SnSe2	O2	−17.5	−3.2	−42.3 (−161.6/~ −40.2)
	H2O	−13.3	+18.0	+220.9
SnSe1.88	O2	−37.6	−26.3	−135.7 (−99.1/−406.7)
	H2O	−27.9	+3.4	+175.6
SnSe	O2	−11.6	−0.2	−236.1 (−323.1/+95.4)
	H2O	−8.1	+23.2	+82.2
SnO2 skin	H2O	−119.7	−106.7	−121.3

Notably, the vibrational spectrum of the oxidized SnSe2 surface (Figure 4a) closely overlaps with the phonon excitation spectrum of SnO2 [56]. Definitely, modes at 48, 99, 126, 177, and 219 meV were measured. In particular, the loss peaks at 48 (A2g phonon) and 99 (B2g phonon) meV are blue-shifted by 4 and 10 meV in the disordered tin-oxide skin formed upon oxidation of SnSe2 compared to their respective value for bulk SnO2 crystals [56].

Figure 4

(a) Vibrational data for oxidized SnSe2(001) (recorded by HREELS with impinging energy of 4 eV) and bulk SnO2(110) (data taken from [56]). (b) Excitation spectrum for pristine and air-modified SnSe2 (recorded by EELS with impinging energy of 300 eV) and bulk SnO2 (data taken from [57]).

The inspection of the excitation spectrum probed by EELS, extended up to the ultraviolet range of the electromagnetic spectrum (Figure 4b), enables monitoring the surface status with a technique with probing depth as low as (0.9 ± 0.1) nm in our experimental conditions [58], which is lower by more than 102 with respect to Raman spectroscopy and optical techniques. Specifically, the excitation spectrum of the as-cleaved SnSe2 surface shows a main feature at 15.9 eV with a shoulder at 12.0 eV, ascribed to interband transitions from Se-4s core levels and, moreover, two weak losses at 7.5 and 26.8 eV. The excitation spectrum of the air-exposed SnSe2 sample is dominated by an emerging broad mode centered around ~18 eV, with two weak peaks at 7.5 and 26.8 eV, evidently insensitive to surface modification. Notably, polycrystalline SnO2 films display the feature at 18.0 eV. Precisely, this feature was previously attributed to the sub-oxide SnO2-x phases [59]. However, the inspection of density of states (DOS) in Figure 5 reveals that the mode at 18.0 eV is related to a single-particle transition starting from O-2s band in SnO2. The weak peaks at 7.5 and 26.8 eV are ascribed to interband transitions originated by Sn-5s and Se-3s levels, respectively.

Figure 5

Partial densities of states for SnSe2 slab with SnO2-skin (see Figure 2d). Fermi energy is set as zero.

Further information on the surface properties was provided by the inspection of core levels by means XPS experiments performed on both pristine and powderized SnSe2 single crystals. As a matter of fact, it is also important to assess the characteristics of the powderized material with the future implementation in devices in mind, for which the surface/volume ratio should be maximized in order to improve the performance. Figure 6 shows the Sn-3d and Se-3d core levels of the as-cleaved SnSe2 single-crystal surface and for the same surface modified by O2 dosage with a total dose of 105 L (1 L = 10-6 Torr·s). The Sn-3d5/2 core level of the as-cleaved sample has a binding energy (BE) of 486.8 eV (Figure 6a). Correspondingly, the Se-3d core levels exhibit a single peak with the J = 5/2 component located at BE = 54.1 eV, compatibly with previous reports for SnSe2 [60] and with a lower BE compared to the case of SnSe (BE = 53.7 eV). Exposure to 105 L of oxygen and storage in air only caused a slight change in the core level of the Se-3d. A new component at BE = 54.7 eV in Se-3d core level arising from Se(0) segregation is observed [61]. The total spectral area of this new component is 5.4% for O2 dosage and 2.6% for air exposure. Especially, from the Se-3d core-level spectra (Figure 6b) one can observe the absence of the SnO2 component, which would be characterized by the J = 5/2 component at BE of ~59–60 eV.

Figure 6

Panels (a) and (b) show Sn-3d and Se-3d core levels for as-cleaved surface of SnSe2 and its modification upon O2 (105 L) dosage and air exposure. The photon energy is 800 eV.

Conversely, in the powderized single crystal, one spectral component arising from surface oxidation was found (Figure 7). Specifically, the new component in the Sn-3d5/2 core level related to SnO2 was observed at BE = 487.8 eV (54% of the total spectral area) [62,63]. Remarkably, even after powderization, no trace of SeO2 is present, as indicated by the featureless Se-3d spectra in the range 59–60 eV [64]. This finding confirms our prediction that Se is only involved in a metastable oxide phase, which represents a precursor for SnO2 formation. However, in the powderized sample, a different Se oxidation state is present, as revealed by a broad feature in the spectra. Precisely, we assign the component at 55.0 eV to Se−2 and the higher to Se−2+δ (0 < δ < 1) [65]. In the powderized sample, we estimate δ to be ~0.15 ± 0.05 from the analysis of the survey XPS spectrum. Therefore, sub-stoichiometric SnSe1.7±0.1 coexists with SnSe2. We also estimated the thickness of the SnO2 surface layer by means of quantitative analysis of XPS data [66], finding a thickness of (0.8 ± 0.1) nm (~2.5 monolayers) without observable changes after an exposure of one week in air. It should be noted that previous reports indicated that the surface of sub-stoichiometric SnSe2 (SnSe1.71) grown by molecular beam epitaxy [67] is unstable, with the subsequent formation of SnOx and SeOx phases. Conversely, in our case, both SnO and SeOx are not present on the surface.

Figure 7

(a) Sn-3d and (b) Se-3d core levels of powderized SnSe2 single crystal. Note that grinding of SnSe2 bulk crystals was carried out in ambient atmosphere. The photon energy is 1486.6 eV (Al Kα).

The SnO2/SnSe2 heterostructure is more sensitive to chemisorbed species with respect to pristine SnSe2. On the pristine surface, the absorption of water molecules generates local changes in charge density in proximity to the absorbed molecules on the surface layer due to a rearrangement of chemical bonds, with a charge transfer of 0.17 e- per water molecule (Figure 8a). Thus, we conclude that pristine SnSe2 is unsuitable for humidity sensing. On the other hand, the absorption of H2O on SnO2/SnSe2 is energetically favorable even above room temperature. We calculated the values of transferred charge from H2O to the SnO2 skin that are 0.43 and 0.30 e− for one and two H2O molecules per supercell, respectively. Correspondingly, the density of states (DOS) is modified with a direct correlation with the coverage of the adsorbate (Figure 8b–d), thus indicating the suitability for humidity sensing, even at the lowest concentrations of H2O. Note that decomposition of water molecule on the SnO2/SnSe2 heterostructure is an exothermic process (−121.4 kJ/mol) and the subsequent water splitting is unfavorable, favoring the reversibility of the process. This further supports the use of the self-assembled SnO2/SnSe2 heterostructure for humidity sensing.

Figure 8

Panels (a-c) report the change of the charge density after adsorption of one water molecule on SnSe2, one water molecule on SnO2-skin-terminated SnSe2, and two water molecules on SnO2-skin-terminated SnSe2, respectively. Panel (d) represents the DOS of SnO2-skin-terminated SnSe2 (black curve) and of the same system modified by the adsorption of one (red curve) and two (blue curve) water molecules. Fermi level is set at 0.

3.3. Gas Sensing

The evaluation of the stability of SnSe2-based systems at the temperature of 200 °C, used in a typical gas sensing experiment, was performed. For the surface model of bulk SnSe2, trilayers of SnSe2 with fixed lattice parameters were used. For free-standing bi- and monolayer optimization of both atomic position and lattice parameters was performed. This provides the contribution in the energetics of the adsorption from flexibility of free-standing few-layers. In order to make calculations more realistic, we also considered the presence of the Se vacancies in the top layer. The formation of the single vacancy turns the SnSe2 to SnSe1.88. Results of the calculations (see Table 2) demonstrate that physical adsorption on bulk and few-layers of SnSe2 is unfavorable at 200 °C. Contrarily, the presence of Se-vacancies makes physisorption favorable. The decomposition of molecular oxygen is favorable in all cases except for monolayer SnSe1.88. This exception is due to the combination of distortions caused by the presence of vacancies and distortions created by formation of new Se-O chemical bonds. Note that this result is valid only for free-standing monolayers. In fact, the deposition of a free-standing monolayer on a substrate decreases the flexibility of SnSex and makes the system closer to a SnSex bilayer.

Table 2

Differential enthalpy (ΔHphys) and Gibbs free energy (ΔG) at an operational temperature of 200 °C for the physisorption of molecular oxygen at the surface of bulk, bilayer, and monolayer SnSe2 and SnSe1.88. The differential enthalphy for oxygen decomposition (ΔHdec) is also reported. All energies are expressed in kJ/mol.

System	Surface	ΔHphys(kJ/mol)	ΔGphys(kJ/mol)	ΔHdec(kJ/mol)
Bulk	SnSe2SnSe1.88	−17.5−37.6	+5.24−14.88	−42.3−135.7
Bilayer	SnSe2SnSe1.88	+38.9−47.6	+61.6−24.9	−76.3−115.5
Monolayer	SnSe2SnSe1.88	+53.1−59.3	+75.8−36.6	−56.3+183.5

The favorability of decomposition of oxygen molecule even at defects-free substrate of SnSe2 could be a starting point for a possible oxidation in the defective areas of SnSe2 (vacancies, edges, grain boundaries). The formation of SnO2 skin on the surface of bulk SnSe2 leads to a migration of Se-atoms to subsurface area with the passivation of Se-vacancies in the sub-surface layers [68]. In the case of free-standing few layers of SnSex, an unstable structure would be formed. Therefore, we exclude this configuration from further investigation. Precisely, the discussed unstable structure is related to few free-standing layers, but if we include the presence of the substrate, the formation of more ordered SnO2 skin will occur. Thus, results obtained for SnSe2/SnO2 could be extended to few layers of SnSe2 deposited on a substrate.

Considering the possibility of the oxidation with the formation of a SnO2 skin, we modeled the physisorption of different analytes on SnSex and on the SnO2 skin over SnSe2 substrate (Figure 9). Results of the calculations (see Table 3) demonstrate that the adsorption of various molecules on SnSex surfaces obeys the same principles as in the case of oxygen. Definitely, defect-free SnSe2 surface is nearly inert for all considered analytes. The presence of Se vacancies turns the free energy in a negative value making the adsorption favorable. Similarly, the adsorption of all analytes on monolayer SnSex is less favorable than the case of bilayer and bulk. In contrast to non-oxidized SnSex substrates, the formation of SnO2-skin corresponds to a strong adsorption of all analytes. The formation of non-covalent bonds between SnO2-substrate and analytes corresponds to an increase of magnitude of the transferred charge (see Figure 8). Note that the magnitudes of the free energies of adsorption and transferred charges in the case of SnO2 skin are much larger than in the case of adsorption on Se-vacancies (SnSe1.88 surfaces). Additionally, in the case of the adsorption on the SnO2 skin, the value of the transferred charge is strongly distinct for different analytes.

Figure 9

Change of charge densities after adsorption of (a,c,e) NH3 and (b,d,f) H2S on (a,b) bulk and (c,d) monolayer of SnSe2 and, moreover, (e,f) the SnO2 skin on an underlying SnSe2 substrate.

Table 3

Differential Gibbs free energies ΔG of physisorption at 200 °C and corresponding values of transferred charge for various combination of considered substrates and analytes

Substrate	Analyte	ΔG (kJ/mol)	Δe−
Bulk SnSe2 (SnSe1.88)	H2H2OH2SNH3NO2	+6.79 (−0.11)+45.63 (−30.97)+38.70 (−21.32)+24.50 (−12.10)+58.87 (−36.75)	+0.09 (+0.10)−0.17 (−0.20)−0.12 (−0.14)+0.06 (+0.12)−0.07 (−0.10)
Bilayer SnSe2 (SnSe1.88)	H2H2OH2SNH3NO2	+8.91 (−0.10)+41.23 (−26.8)+36.14 (−29.65)+18.91 (−10.01)+46.75 (−25.44)	+0.08 (+0.10)−0.15 (−0.18)−0.10 (−0.13)+0.10 (+0.15)−0.10 (−0.11)
Monolayer SnSe2 (SnSe1.88)	H2H2OH2SNH3NO2	+2.89 (−4.63)+28.97 (−7.11)+18.35 (−0.12)−4.91 (−12.10)+0.12 (−3.45)	+0.10 (+0.09)−0.15 (−0.20)−0.12 (−0.09)+0.16 (+0.15)−0.09 (−0.10)
SnO2/SnSe2	H2H2OH2SNH3NO2	−135.61−60.80−71.28−96.20−127.35	+0.15−0.43−0.36+0.44−0.25

The theoretical model discussed above was confirmed by experiments. The sensing properties of SnO2/SnSe2 heterostructures were measured by monitoring the resistance change upon exposure to different concentrations of gases. In Figure 10, the dynamic response-recovery curve with NH3 (40 ppm) and NO2 (1 ppm) is shown, with opposite signs in the response curve related to the opposite charge transfer, as depicted in the inset. Interestingly, sub-ppm limit of detection is feasible for both cases, with 250 and 400 ppb for NH3 and NO2, with near-room-temperature operational temperatures.

Figure 10

Dynamic response-recovery curve for NO2 (1 ppm) and NH3 (40 ppm). The inset depicts that electronic charge transfer occurs from SnSe2 to NO2, while the opposite charge transfer exists in the case of NH3, congruently with experimental findings on electrical tests. Reproduced with permission from [79].

The SnO2/SnSe2 heterostructure exhibit significantly enhanced response and superb response/recovery characteristics, also for H2S, for which the lowest detection limit reaches 10 ppb with a response value of 2, which is below the acceptable ambient levels of 20–100 ppb set by the Scientific Advisory Board on Toxic Air Pollutants (USA) [69,70].

Similarly, H2 detection is feasible using SnO2/SnSe2 heterostructure [68] with a response of 3 with an operational temperature of 150 °C at a concentration of 100 ppm and a limit of detection of 5 ppm. Considering the quite reduced costs of raw elements, this could be relevant considering that in the growing market for H2-powered devices—i.e., fuel cells—there is a requirement for cheap H2 sensors.

Table 4 reports an overview of literature results on near-room-temperature sensing of H2, H2S, NH3, and NO2 for SnSe2- and SnO2-based sensors, at their respective operational temperature and concentration. Sensitivities of 0.43, 2.13, 0.11, and 1.06 [ppm]−1 were reported for H2, H2S, NH3, and NO2, respectively (Table 5, also reporting the limit of detection).

Table 4

Sensing of H2, H2S, NH3, and NO2 for SnSe2- and SnO2-based systems. The response obtained at the respective operational temperature and gas concentration is reported. RT means room temperature.

Gas	Sensing Materials	OperationalTemperature (°C)	Concentration (ppm)	Response	Reference
H2	SnO2/SnSe2-x	150	100	3	[68]
H2	SnO2	150	1000	5.5	[75]
H2S	SnO2	100	10	1–6	[76]
H2S	SnSe2	RT	10	10–15	[77]
H2S	SnO2/SnSe2	RT	10	32	[70]
H2S	SnO2	RT	50	33	[78]
NH3	SnO2/SnSe2	RT	100	2	[70]
NH3	SnSe2	RT	40	2.7	[79]
NH3	Au-SnSe2	RT	5	5.3	[80]
NO2	SnO2/SnSe2	RT	10	3.5	[70]
NO2	SnO2/SnSe2-x	150	1	3.2	[68]
NO2	SnSe2	RT	1	6	[79]
NO2	SnSe2	RT	5	112	[81]
NO2	SnSe2/SnSe	RT	1	75	[82]
NO2	Au/SnSe2	130	8	3	[83]
NO2	Pt-SnSe2	130	8	3.9	[83]
NO2	SnSe/SnSe2	RT	5	12	[84]
NO2	SnSe2	RT	8	1.4	[85]
NO2	SnO2	100	10	1	[76]

Table 5

Sensitivity to H2, H2S, NH3, and NO2 for SnSe2-based sensors, as well as their limit of detection.

Gas	Sensitivity [ppm]−1	Limit of Detection
H2	0.43 ± 0.02 [68]	5 ppm at 150 °C [68]
H2S	2.13 ± 0.01 [77]	10 ppb at RT [70]
NH3	0.11 ± 0.01 [80]	250 ppb at RT [80]
NO2	1.06 ± 0.03 [68]	400 ppb at 150 °C [68]

The SnO2/SnSe2 heterostructure exhibits higher performance in terms of high response to analytes, low detection limit, high selectivity, and low power consumption [69,70,71,72,73,74]. Therefore, based on both calculations and experiments, one can conclude that the SnO2/SnSe2 heterostructure is a promising platform for gas sensing.

3.4. SnSe2-Based Sensors for Large-Area Imaging with Millimetre Waves

An efficient THz detector at room temperature needs two characteristics: high mobility and a channel with nanometer length in a FET. Alternatively, the electron heat-driven detection mechanism could be used [86], based on the specific geometry of the device and the thermal/electrical properties of the material. In fact, SnSe2 has both a high thermoelectric power and a suitable bandgap [87], which are advantageous characteristics for the production of hot electrons and for photothermal conversion. To improve the THz photodetection performance of the SnSe2-based device, the relationship between channel length and photoresponse should be investigated. In FETs, an important issue concerning THz detection is that the channel length should be less than tens of nanometers at the expense of rectification ratio. As mentioned above, it is possible to exploit hot electrons to overcome this limit, related to the absorption efficiency of photons. Specifically, the performance of SnSe2-based THz detectors with a channel length of 6 µm and 100 nm, along with the simulated electric field intensity, were evaluated (Figure 11a). A short channel SnSe2 device is reported in Figure 11a-i,ii. In Figure 11a-iii, one can notice how the reduction of the antenna gap increases the value of the square of the electric field intensity (E02) in the channel by almost three orders of magnitude. To study the different photoelectric properties of the devices as a function of the channel length, the photocurrent, the response time, and the reactivity were measured simultaneously using two radiations 0.04 and 0.12 THz with a power density of 2.5 and 1 mW cm−2, respectively. The photocurrent increases linearly under electrical bias as VDS from −0.1 to 0.1 (Figure 11b), due to the increased carrier drift-velocity and reduced carrier transit time. Moreover, the short channel devices show a photocurrent greater than three orders of magnitude at 0.04 THz, congruently with the theoretical predictions of Figure 11a-iii. By exploiting the antenna gap in short channel device, it is possible to concentrate the THz radiation in a very small spot, consequently improving the SnSe2 intraband absorption capacity, thus hot carriers can be efficiently produced [88]. In Figure 11c, one can observe a good linearity over a wide range of the photocurrent, of the short channel device, as a function of the incident power. Using the equation Iph ∝ Pβ to fit the experimental data, β is ~0.98 under positive or negative bias, which is an interesting value for high-contrast imaging. Even at the frequency of 0.12 THz, there is the same improvement effect, as evident from Figure 11d, confirming the broadband nature of THz field nano-focusing. Additionally, the response time at 0.12 THz for both short and long channel devices was also studied (Figure 11e). For the short channel device, τrise =2.7µs and τfall = 2.3 µs—i.e., a response time of approximately 16 times faster than in the long channel device (τrise = 45.4 µs, τfall = 46.5 µs). Obviously, the long channel device has a slower response time than the short channel device, making the short channel device advantageous in applications. The increase in VDS rapidly increases the bias voltage response to 0.12 THz (Figure 11f). For short channel devices, the maximum response is 2.5 A W−1, which is approximately five times larger than graphene-based devices (20 V W−1) [89] and 12 times larger than black-phosphorus-based devices (7.8 V W−1) [90]. This indicates that short-channel SnSe2 devices have superior performance respect to graphene- and black phosphorus-based devices.

Figure 11

(ai) Schematization of the realization of the short-channel device. (aii) Image of a short channel SnSe2-based photodetector acquired with the SEM. (aiii) The electric field distribution of the SnSe2-based long channel (left panel) and short channel (right panel) photodetector. (b) and (c) Photocurrent as a function of the bias voltage and incident power at 0.04 THz, respectively. (d) Photocurrent as a function of bias voltage at 0.12 THz. (e) (top panel) Long channel and (bottom panel) short channel time-resolved photocurrent devices at 0.12 THz with 1 mW cm−2 power. (f) Photoresponsivity vs. bias voltage at 0.12 THz with 1 mW cm−2 power. Reproduced with permission from [91].

Exploiting the excellent performance of the SnSe2, a large area imaging device with THz transmission was created for identifying hidden objects. Using a radiation of 0.12 THz, the shape of the glue-jar in the box was clearly visible (see Figure 12b). Furthermore, it was possible to detect not only the shape of the glue-jar but also the position of the amount of the glue inside the jar. Another key feature of the THz detector is its stability. Differently from black phosphorus [92], SnSe2-based devices exhibited better stability, congruently with the ambient stability assessed in Figure 6 by surface-science techniques. The absence of noticeable modifications in the photocurrent at 0.12 THz radiation after a prolonged storage in air (extended up to one month, Figure 12a) corroborates the excellent stability of SnSe2-based optoelectronic devices.

Figure 12

(a) Aging in air of SnSe2-based short channel photodetector, black and purple spot represent the photocurrent of as-prepared device and after 30 days in air, respectively. (b) The THz imaging of a glue-jar in the box at 0.12 THz. Reproduced with permission from [91].

4. Conclusions

Here, we elucidated the main features of chemical reactivity of SnSe2. While the stoichiometric single crystal is chemically inert to ambient gases, the presence of selenium vacancies drastically affects surface chemical reactivity. The SnSe2−x surface is transformed into SnO2-skin-terminated SnSe2, with a thickness of the SnO2 skin estimated to be sub-nanometric.

Interestingly, while the self-assembled SnO2/SnSe2-x heterostructure is an exceptional platform for detecting chemical species, as demonstrated for H2, H2O, NO2, NH3, and HS2, the pristine SnSe2 is unable to detect the same species. Thus, our results highlight the pivotal role of Se vacancies in metal dichalcogenides, which can transform the system from ambient-stable to an ultrasensitive gas sensor by tuning the stoichiometry.

Sensitivities of 0.43, 2.13, 0.11, and 1.06 [ppm]−1 were reported for the detection of H2, H2S, NH3, and NO2, respectively. The corresponding limit of detection is 5 ppm, 10 ppb, 250 ppb, and 400 ppb for H2, H2S, NH3, and NO2, respectively.

Moreover, SnSe2 is particularly suitable for THz photodetection, based on hot electrons. The response speed and the reactivity of the device are significantly improved thanks to the short channel, which exploits the localization of the electrostatic field and the high thermoelectric value of SnSe2. Furthermore, the device has excellent stability, even when the uncapped active channel is exposed to air for long periods, thanks to the exceptional chemical inertness of the single stoichiometric SnSe2 crystals. Accordingly, SnSe2-based photodetectors represent suitable and promising candidates for imaging applications for homeland security and quality controls.