CrI3-WTe2: A Novel Two-Dimensional Heterostructure as Multisensor for BrF3 and COCL2 Toxic Gases

A new multisensor (i.e. resistive and magnetic) CrI3-WTe2 heterostructure (HS) to detect the toxic gases BrF3 and COCl2 (Phosgene) has been theoretically studied in our present investigation. The HS has demonstrated sensitivity towards both the gases by varying its electronic and magnetic properties when gas molecule interacts with the HS. Fast recovery time (<0.14 fs) under UV radiation has been observed. We have considered two configurations of BrF3 adsorbed HS; (1) when F ion interacts with HS (C1) and (2) when Br ion interacts with HS (C2). In C1 case the adsorption energy Ead is observed to be −0.66 eV while in C2 it is −0.95 eV. On the other hand in case of COCl2 Ead is found to be −0.42 eV. Magnetic moments of atoms are also found to vary upon gas adsorption indicates the suitability of the HS as a magnetic gas sensor. Our observations suggest the suitability of CrI3-WTe2 HS to respond detection of the toxic gases like BrF3 and COCl2.

structure in 2D materials can be altered [28][29][30] . Integrating such thin magnetic materials with the non-magnetic semiconductor actually induces a two-sided effect: (1) magnetic material affects the magnetic property of the magnetic-semiconductor interface that ultimately modifies the electronic structure of semiconductor and, (2) the semiconductor affects the magnetism of the magnetic material. Such a combination of materials produces highly sensitive heterostructure that can alter its magnetic or electronic properties upon adsorption with foreign molecules. In this paper, we have investigated one such atomically thin magnetic material CrI 3 monolayer integrated over WTe 2 monolayer to form a CrI 3 -WTe 2 heterostructure (HS) material.
The BrF 3 is a hazardous gas used mainly in the processing of nuclear fuel. It is corrosive to metals and tissues and irritates the respiratory upon inhalation. It is a powerful oxidizer and highly reactive and a corrosive gas which can cause severe damage to the body like its contact can severely burn skin and eyes. On the other hand, phosgene is highly toxic gas used in industries for the production of pesticides and its immediate reaction starts even below 2-3 ppm. There are extensive reports on literature for COCl 2 gas sensing and have shown better performance 31,32 . But, so far no investigation has been done on BrF 3 gas adsorption on sensor layer. Thereby, it requires proper detection and thus necessitates a sensor that could do so.
Generally, the gas sensing mechanism is based on the principle of change in electronic properties with gas adsorption. Variation in magnetic properties upon gas adsorption has never been realized. Here we studied the gas sensing ability of a CrI 3 -WTe 2 HS upon interaction with noxious gases BrF 3 and phosgene (COCl 2 ). Thus this paper focuses on the study of how the gas molecules (BrF 3 and COCl 2 ) interfere with the electrical and magnetic properties upon interaction. We have also investigated the nature of adsorption and selectivity towards each gaseous molecules. Practically, a sensor's recovery time (R T ) is crucial for technological applications, thus R T for the highly selective gas molecule is calculated for this system.

computational Details
At ambient temperature and pressure conditions, the crystal structure of CrI 3 -WTe 2 HS is shown in Fig. 1a. The results presented here are obtained using the first-principles approach which based on density functional theory 33 as implemented in Quantum Espresso package 34 . Ideally, CrI 3 exists in two crystal structures: (1) AlCl 3 type monoclinic array and (2) BiI 3 type rhombohedral order 35 . Here we report our findings for monoclinic assembly of CrI 3 deposited over hexagonal structure of WTe 2 27 . In order to explore the electronic structure of pure and BrF 3 /phosgene gas adsorbed CrI 3 -WTe 2 HS, we have employed plane-wave ultrasoft pseudopotential method to trace the valance electron interactions. To serve the exchange-correlation potential, generalized gradient approximation (GGA) of Perdew-Burke-Erzernhof (PBE) 36 has been implemented. A supercell of 2 × 2 × 1 has been used to construct CrI 3 -WTe 2 HS. The cut-off kinetic energy of 760 eV has been applied with 7 × 7 × 1 K-mesh for Brillouin zones sampling. We have used these values after complete optimization process. To avoid any interaction among atomic orbitals we have provided a large vacuum of 17 Å along z-direction. The CrI 3 -WTe 2 HS has been allowed to fully relax under the convergence of total energy and total forces which are found to be better than 1.0 meV. For gas sensing calculations, we have kept the structural geometry of CrI 3 -WTe 2 HS fixed and periodically moved the gas molecules BrF 3 and Phosgene COCl 2 (one at a time) along z-direction in order to acquire the equilibrium distance d eq between the HS and gas molecule. In case of BrF 3 gas molecule, we have studied its interaction with the HS along two different orientations (1) F atom is interacting with HS surface and (2) Br atom is interacting with HS surface. The value of d eq obtained in BrF 3 in case 1 is ~2.25 Å whereas in case 2 it is observed to be ~2.04 Å. shows the total ground state energy of gas molecule adsorbed HS. It is worth to mention that the adsorption of gas molecules with the HS is attributed to surface adsorption hence the gas molecules will interact only with the CrI 3 top surface layer.

Results and Discussion
In the present investigation, we have studied the HS which is comprised of a monolayer of CrI 3 deposited over the honeycomb WTe 2 monolayer. Because of the presence of magnetic Cr 3+ ion in CrI 3 layer of HS, we have first carried out the calculation for two different magnetic configurations namely, ferromagnetic (FM) and antiferromagnetic (AFM) spin states at Cr site. Since the AFM state gives higher energy as compared to FM one, thus FM configuration is the stable magnetic state which is in accordance with the previous reports 35 . Hence, further investigations have been done for FM configuration only. Fig. 1a is composed of WTe 2 and CrI 3 monolayers. From Fig. 1a we can see that in WTe 2 , the W ions form a zig-zag pattern along a-axis resulting in slightly distorted hexagonal symmetry. The Te ions constitute an octahedral environment accompanied by strong intra-layer covalent bonding w.r.t. W ions. Whereas, in the CrI 3 layer of HS, Cr 3+ ions form a honeycomb lattice. The I-ions create an edge sharing octahedrally coordinated network w.r.t. Cr 3+ ions such that the three I-ions are coordinated at the top and bottom layer of Cr ions. The two parent compounds (WTe 2 and CrI 3 monolayer) are vertically stacked together along c-axis to form a CrI 3 -WTe 2 HS with interfacial bonds linking I and Te ions. The 〈Te−I〉 average bond length at the interface is 2.61 Å. An overall compression along c-axis has also been observed among the parent compounds of the HS which may affect its electronic structure. Table 1. displays the comparison of experimental and calculated bond length in HS. From the results obtained in Table 1 we observed a net compression in the HS (i.e. <45%) except for 〈W−W〉 (~11%) due to its relatively heavy atomic mass which obstructs any significant variation in its bond length as compared to other atoms. Hence the compression is emerging due to the interfacial bonds formed among the parent compounds of the HS (i.e. WTe 2 and CrI 3 ). These bonds are occurring from the charge transfer from Te ions to I ions (charges flows from low electronegativity (Te = 2.1 Pauling scale) to high electronegativity (I = 2.6 Pauling scale)). This process of bond formation at the interface of HS, in turn, results in compression of bond length among the atoms upon optimization. The electronic properties of the HS may get influenced due to this compression which has been discussed in detail in the following section. The interaction of BrF 3 on HS can occur through two possible orientations: by forming an interfacial bond between (1) F and HS as shown in Fig. 1b (C1 configuration), (2) Br and HS as shown in Fig. 1c (C2 configuration). The 〈F−Br−F〉 bond angle is 86° with the 〈Br−F〉 bond length along an axial and equatorial plane as 1.72 Å and 1.81 Å respectively. The equilibrium distance (d eq ) in C1 and C2 case is 2.25 Å and 2.04 Å respectively. For the COCl 2 gas, the bond angle and bond length is 124° 〈Cl−C−O〉 and 1.76 Å 〈Cl−C〉, 1.19 Å 〈C−O〉 respectively. Unlike BrF 3 , only single orientation of COCl 2 has been considered (Cl linked with HS) as presented in Fig. 1d  electronic structure. In order to investigate the gas sensing effect of the HS, we have first studied the electronic density of states (DOS) prior to the gas adsorption. When no gas molecules were adsorbed, the total DOS of HS (Fig. 2a) shows a spectral weight of 11.83 states/eV at the Fermi level (FL). Small amount of metallicity is induced because of Cr-3d and I-5p orbitals of CrI 3 layer of HS. This induced metallicity is emerging from the compressed bond length of the atoms upon optimization as discussed above. Whereas the WTe 2 counterpart displays an insulating behavior with a gap of 1.31 eV between majority and minority spin channels. Experimentally, CrI 3 layer is insulating in nature 35 but in CrI 3 -WTe 2 HS, half-metallicity is observed. This might be due to the electron doping of CrI 3 layer induced by WTe 2 as reported previously 37 . Near FL (E = −0.48 eV) only contributions from W-5d orbital and Te-5p orbital dominates whereas Cr-3d and I-5p orbital state lies 1.32 eV below FL. For pristine HS the total bandwidth for metallic state is observed to be 0.02 eV (Fig. 2a) with the majority spin carriers separated from minority spin carriers by 0.88 eV. www.nature.com/scientificreports www.nature.com/scientificreports/ BrF 3 adsorbed HS. When BrF 3 gas molecule is adsorbed on the HS the metallicity is enhanced in both the orientations which can be seen from Fig. 2b,c. In C1 configuration (Fig. 2b), when F directly forms an interfacial bonding with HS, the bandwidth increases to 0.67 eV. At FL, the dominant contribution is coming from Cr-3d states with the spectral weight for up and down spin density being 0.85 states/eV and 1.56 states/eV respectively. Feeble participation of W-5d (up 0.33 states/eV, down 0.77 states/eV) and Te-5p (up 0.14 states/eV, down 0.21 states/eV) and I-5p states(up 0.23 states/eV, down 0.37 states/eV) are also observed at FL. The adsorbed BrF 3 gas molecule in C1 have enhanced spin up DOS at FL. In principle, the charges should flow from Br (low electronegativity) to F (high electronegativity) ions but due to the large 〈Br−F〉 bond length (<1.7 Å) the charge hopping takes place at slower rate resulting in higher spectral weight of Br (2.08 states/eV) ion as compared to F (0.97 states/eV) ions at FL. On the other side for C2 configuration (Fig. 2c), when Br interacts directly with HS, the bandwidth further intensifies to 0.76 eV at FL. Likewise in C1, here also the Cr-3d states are pronounced at FL with spectral weight of 1.25 states/eV for all spin channels. Relatively weak involvement of W-5d (0.95 states/eV), Te-5p (0.2 states/ eV) and I-5p (0.42 states/eV) states are present at FL. Now since the electronegativity of I, Br and F are 2.66, 2.96 and 3.98 Pauling scale respectively, therefore a continuous flow of the charge will take place from I to F via Br ion. Hence, net charge density at Br site decreases as compared to the previous case. In C2, the net charge transport takes place through I-Br-F chain which facilitates the smooth transfer without any accumulation while the charges stocked at Br site due to uneven path (I-F-Br) w.r.t. electronegativity in C1. To investigate the nature of bonding between BrF 3 and HS, we have studied the charge density for both the orientations (Fig. 3a,b). For this purpose, the calculations were performed in (1 1 0) plane. As discussed earlier that the charge transport channel is decided by the electronegativity difference. In C1 (Fig. 3a), electronegativity of WTe 2 , CrI 3 and BrF 3 is 1.5071, 2.6980 and 4.2333 respectively where the maximum charge is accumulated near F (4.2333) ion of BrF 3 . Hence, a net flow of charge will take place from WTe 2 to BrF 3 layer via CrI 3 . Similar charge flow network is followed in C2 as shown in Fig. 3b where electronegativity of WTe 2 , CrI 3 and BrF 3 is observed to be 1.5067, 2.6974 and 4.2323.

Exp (Å) Calculations (Å)
Phosgene (COCl 2 ) adsorbed HS. Another poisonous gas COCl 2 , when adsorbed on the HS surface, exhibits a gapless type semiconducting behavior which can be seen in Fig. 4a showing the PDOS of the above stated system. With adsorption, the states are present near the FL but do not cross for W and Te ions. A similar situation is observed for Cr-3d and I-5d states. This is in contrast with BrF 3 adsorption where the metallicity was induced at each HS layer. Within −0.26 eV-0.31 eV energy window, the HS is occupied by the states of both spin channels near FL whereas, negligible contributions are coming from the gaseous states in that energy range. In the COCl 2 www.nature.com/scientificreports www.nature.com/scientificreports/ gas molecule, the electronegativity of C, Cl and O are 2.55, 3.16 and 3.44 Pauling scale respectively. Thus, the charge flow from C will take place to O and Cl ions resulting in the slight occupation of states below FL in O and comparatively higher states at Cl site. The DOS of Cl ions (Fig. 4a) has the 3p states peaked within 0.5 eV-0.7 eV below FL due to higher charge density coming from C and HS layers. From the charge density calculations we observed a charge flow direction from HS to the gas molecule as given in Fig. 4b. The electronegativity of WTe 2 , CrI 3 and COCl 2 is 1.5075, 2.6987, 4.2343 respectively. It results in a unidirectional charge flow from bottom of HS to the gas molecule which causes the accumulation of carriers at COCl 2 . The above observations suggest that CrI 3 -WTe 2 HS serve as a potential gas sensor for BrF 3 and COCl 2 gas molecule.

Magnetic properties.
In HS the magnetic contribution is coming due CrI 3 layer which has FM ordering with the total spin magnetic moment of 5.35 μ B . And the magnetic moment per Cr and I ions are 2.86 μ B and 0.041 μ B respectively. This is in good agreement with the saturation moment(3.1 μ B /Cr) measured experimentally 35 . The low magnetic moment of I ions is due to the transfer of unpaired 4 s electron from Cr to I-resulting in stable 5p states. This delocalization of charges causes a reduced moment at I-site. On the other hand, the WTe 2 layer in HS remains non-magnetic. With the exposure of BrF 3 gas molecule in C1 configuration, the delocalization effects dominate in Cr ions resulting in decreased magnetic moment. The Br and F ions, on the other hand,  www.nature.com/scientificreports www.nature.com/scientificreports/ acquire charges from HS have higher magnetic moment (0.088 μ B for Br and 0.005 μ B , 0.144 μ B , 0.0268 μ B per F ion) as compared to Cr 3+ (0.009 μ B ). As discussed above that due to the difference in 〈Br−F〉 bond lengths uneven charge flow takes places to form dissimilar magnetic moment per F ion. In C2 configuration when Br directly bonded with HS layer total magnetic moment is almost negligible. This is in accordance with the DOS of C2 (Fig. 2c) where the net reduction in spectral weight was observed. Due to the continuous charge transfer path (I-Br-F) the delocalization of electrons causes the moments to drop. The same scenario has been observed when Phosgene is exposed to HS. The overall reduction in the magnetic moment is observed here as well. We have tabulated (in Table 2) the % change in density of states of HS at Fermi level (Δρ) and magnetic moments (Δm) with respect to no adsorption of gas for all the cases. The variation in magnetic moments of the HS upon the interaction of gas molecules suggests that CrI 3 -WTe 2 HS can be used as a magnetic gas sensor as well as a resistive gas sensor. There are many studies over the latter type but only a few experimental studies are performed on the former type of gas sensor which detects the perturbation in the magnetic properties when gas molecules interact with the sensor material 38 . A few magnetic gas sensors so far studied are nanoparticles of CuFe 2 O 4 which was used for the detection of volatile organic compounds (VOCs) 39 , Co/ZnO nanorods to detect H 2 and CO molecules 40 , Co/ZnO hybrid nanostructures for the detection of C 3 H 6 O, CO and H 2 target gases 41 etc. Some of the experiments are mainly devoted to hybrid ferromagnetic/semiconducting materials such as Co/ZnO instead of common dilute magnetic semiconductors as the later requires high magnetic field for their application. In references (Ponzoni et al. 40 and Ciprian et al. 41 ) it has been found that in hybrid Co/ZnO system, the change in magnetization is linearly dependent on the concentration of gas. Thus, the higher the concentration of the gases, the higher would be the magnetization change. In our case, the system taken is also a hybrid one. In this we took a single gas molecule for the study, hence small changes in the magnetization have been observed.
Adsorption and recovery time. The adsorption energy describes the nature of stability among adsorbent (HS) and adsorbate (gas molecule). The process can take place in two modes (1) physisorption: which involves weak van der Waals forces between two reacting species. The electronic properties of adsorbent are barely perturbed during this mechanism; (2) chemisorption: here actual involvement of chemical bonds between species exists. This also requires minimum activation energy to initiate the process. In C1 with BrF 3 adsorption on HS, the adsorption energy is −0.66 eV while in C2 it increases to −0.95 eV. The increasing adsorption rate by 30% suggests the comparatively strong chemisorptive nature in C2. The existence of strong covalent bonding between HS and BrF 3 (C1 and C2) have been observed from charge density results shown in Fig. 3a,b. Similar studies for COCl 2 adsorption shows the chemisorptive character but the E ad (−0.42 eV) is relatively smaller than that from BrF 3 interaction. It is evident that as d eq increases, E ad energy decreases. Thus, the higher d eq for COCl 2 case is marked by a decrease in E ad . Though the relative stability in COCl 2 case is lesser than BrF 3 but from previous literature COCl 2 adsorption on BN nano tube (BNNT), BN nano rod (BNNR) and borophene reported to have E ad −0.18 eV, −1.058 eV and −0.306 eV respectively 42,43 . Hence, COCl 2 adsorption on CrI 3 -WTe 2 HS has shown better performance as compared to previous reports with an exception of BNNR.
The recovery time R T of a sensor is based on how fast the sensor retrieves its initial state. Based on the Arrhenius theory the sensor recovery time 44 is related by: where ν is the operational frequency, E ad is adsorbate energy, K is Boltzmann constant and T is the sensor's operational temperature. For different attempt frequencies, the sensor's recovery rate is affected as tabulated in Table 3. We have used one frequency each lying on IR, visible and UV range to calculate the corresponding R T in each case. We believe that by changing frequency inside a specified range (ie., IR, visible and UV), the order of R T   www.nature.com/scientificreports www.nature.com/scientificreports/ will remain the same. The calculations are done for room temperature (300 K). Under UV illumination the HS is showing faster R T for all the cases. The recovery rate depends on the nature of adsorption. With the relatively weak chemisorptive effect of COCl 2 gas on HS, fastest recovery time (R T ) is achieved.

conclusions
We have theoretically investigated a new 2-dimensional CrI 3 -WTe 2 HS in the present work in order to explore the possibility as a multisensor (i.e. resistive and magnetic). Our results show that upon interaction with the gas molecules BrF 3 and COCl 2 with HS the electronic as well as magnetic properties of pristine HS get altered. We have also determined that how swiftly the HS can get recover after detaching the gaseous species from it by means of recovery time. We found that under UV illumination ultrafast recovery time is presented by the HS i.e. <0.14 fs. Hence we conclude that CrI 3 -WTe 2 HS offers itself as a multisensor for the detection of highly toxic gases like BrF 3 and COCl 2 .