Monolayer Sc2CF2 as a Potential Selective and Sensitive NO2 Sensor: Insight from First-Principles Calculations.

Two-dimensional materials with excellent surface-volume ratios and massive reaction sites recently have been receiving attention for gas sensing. With first-principles calculations, we explored the performance of monolayer Sc2CF2 as a gas sensor. We investigated how molecule adsorption affects its electronic structure and optical properties. It is found that a large charge transfer quantity happens between Sc2CF2 and NO2, which results from the fact that the lowest unoccupied molecular orbital (LUMO) of NO2 is below the valence band maximum (VBM) of Sc2CF2. Moreover, the MD simulation shows that NO2 can adsorb on the Sc2CF2 surface stably at room temperature. We explored the effect of biaxial strain on the adsorption energy and charge transfer quantity of each system, and the results show that the biaxial strain can enhance both the adsorption energy and charge transfer quantity of the NO2 system and thus can improve the sensitivity of Sc2CF2 in detecting the NO2 molecule. Furthermore, we investigated the adsorption behavior and charge transfer of polar polyatomic molecules at the Sc2CF2 surface with h-BN as a substrate, and the results demonstrate that the h-BN substrate can hardly modify the main results. Our result predicts that Sc2CF2 can be a promising selective and sensitive sensor to detect the NO2 molecule, and could also give a theoretical guide for other terminated MXenes used for gas sensors or detectors.

Introduction

Gas sensors and detectors play an important role in environmental protection and daily health safety applications. Because of the advantages of low costs, metal oxides have been explored widely; however, they require a high operating temperature.[1,2] With excellent surface–volume ratios and massive reaction sites, a large number of 2D materials including graphene,[3−5]h-BN,[6] phosphorene,[7−10] transition-metal chalcogenides,[11−13] and layered group III–VI semiconductors[14−17] have been reported with superior performance for detecting gas.[18−21] Charge transfer happens when molecules are adsorbed on 2D materials, and the charge transfer between molecules and 2D materials will modify the electronic structures and/or resistance of sensing materials by varying degrees. A copious amount of charge transfer is required for high selectivity and sensitivity 2D sensing materials.

MXenes, including traditional metal carbides, nitrides, and carbon-nitrides with possible stoichiometric ratios of 2:1, 3:2, and 4:3, are a large family of 2D materials,[22−24] which are usually synthesized through chemical exfoliation from the MAX phase by etching the ″A″ atoms in acid solutions. Meanwhile, the intrinsic metallic surface of MXenes is usually passivated or terminated by a function group, and there consist four passivation models depending on function group adsorption sites on both sides.[25−27] With a natural layered structure, MXenes have been explored as high-performance electrode materials in lithium-ion batteries.[28−32] For instance, bare Ti3C2 shows excellent theoretical capacity and low Li-ion diffusing barrier,[30] and the rate performance of terminated Ti3C2 can be greatly enhanced after intercalation.[31,32] MXenes also find potential applications in other energy storage aspects, like supercapacitors,[33−35] Li-S batteries,[36] and multivalent-ion batteries.[37−39] Owing to their intrinsic metallic property, MXenes flakes, which have low average resistivity and high electron mobility, also have been reported to be used as a conductive channel in field-effect transistors, which may construct an Ohmic contact.[40,41] Besides, low-thickness Ti2CX2 films with a transmittance of about 90% have been explored as transparent conductive electrodes.[42−44] It has also been reported that active sites for water redox exist in MXenes, which also show suitable Gibbs free energy for hydrogen adsorption and are electro-catalyst or photo-catalyst candidates for water splitting.[45−47] MXenes have found promising application in various fields.

Besides, MXenes also have been explored as sensing materials for sensors or detectors. In terms of theoretical calculations, MXenes have been widely explored for gas sensing. Density functional theory (DFT) calculations demonstrated that bare MXenes are reactive, where molecules’ dissociative adsorption happens at most surfaces, and the selectivity is enhanced upon surface functionalization.[48] DFT calculations along with non-equilibrium Green’s function (NEGF) method simulation predicted that O-terminated Ti2C can detect NH3 sensitively with a strong binding energy. A dramatic change of the I–V curve after NH3 adsorption determines the selectivity of O-terminated Ti2C.[49] In addition, applied strain can increase the binding energy and improve the sensitivity, while releasing the strain or injecting an electron into MXenes will weaken the strength and achieve reusability of sensing materials.[49,50] Similarly, it has been reported that SO2 can be detected by Sc2CO2 with suitable adsorption energy. Moreover, biaxial strain and external vertical electric field can help SO2 desorption from Sc2CO2.[51] In terms of experiments, Wang et al. have reported that stacked Ti3C2TX molecular sieving membranes exhibit excellent H2 permeability, which mainly benefit from the sub-nanometer interlayer spacing between the neighboring MXene nanosheets.[52] Stanciu et al. designed a nanohybrid structure with MXene (Ti3C2TX) and transition metal dichalcogenide (WSe2), which can adsorb oxygen species (O2– and O–) in air and exhibits good flexibility for O-containing volatile organic compounds with low noise and fast response/recovery speed.[53]

To our best knowledge, MXenes are potential candidates for gas sensors. Here, we performed DFT calculations to explore the adsorption of 11 common molecules on F-terminated Sc2C (Sc2CF2) and their charge transfer, which have not been studied yet. We investigated how molecule adsorption affects their electronic structure and optical properties. It is found that the largest charge transfer quantity happens between Sc2CF2 and NO2, which is because the lowest unoccupied molecular orbital (LUMO) of NO2 is below the valence band maximum (VBM) of Sc2CF2, although the adsorption energy is not very large. The adsorption stability and sensitivity of Sc2CF2 sensing NO2 are demonstrated through molecular dynamics simulation with an NVT ensemble, and applying biaxial strain is claimed to be a promising method for enhancing the sensitivity and selectivity. Besides, h-BN as a substrate can hardly modify the adsorption energy and charge transfer at the polar polyatomic molecules’ adsorption system and is a promising substrate. These results indicate that F-terminated Sc2C is a good candidate for a NO2 gas sensor, and give a theoretical guide for other terminated MXenes used for gas sensors or detectors.

Results and Discussion

To simulate the Sc2CF2 monolayer exposed in air, we place 11 common molecules on the surface of Sc2CF2, and several adsorption configurations, i.e., top, bridge, and hollow sites of the three top F atoms, as well as the vertical and parallel orientation of each molecule, are considered. To characterize the interaction strength between the molecule and the 2D sheet, we define the adsorption energy (Eads) as Eads= Etot – E2D – Egas, where Etot and E2D are the energies of the monolayer Sc2CF2 with and without adsorption of a gas molecule, respectively; Egas is the energy of a gas molecule in a cubic lattice with a lattice constant of 15 Å. The most stable configurations and Eads for each molecule system are shown and listed in Figure and Table . The homo-nuclear diatomic molecules, like H2, O2, and N2, will stay parallel to the substrate, and the weakest interaction with Eads ranging from −0.07 to −0.09 eV per molecule is found in these cases. The polar hetero-nuclear diatomic molecules, like NO and CO, also remain parallel with Sc2CF2, and the Eads is also small; however, it is a little stronger than that of the homo-nuclear diatomic molecule (Eads = −0.13 and – 0.14 eV for NO and CO, respectively), which is due to the strong electronegativity of the surface F atoms of the substrate. The CO2 molecule has no polarity and contacts weakly with Sc2CF2. For polar polyatomic molecules (H2S, H2O, SO2, NO2, and NH3), the adsorption is much stronger. The Eads of H2O and H2S is −0.20 and – 0.18 eV per molecule, respectively, and the H in both cases will favor staying at the top of F, while the O in H2O tends to stay at the top of Sc and S prefers the top of C. The NO2 is more likely to stand vertically on the substrate, while SO2 prefers to lie parallel on the surface. In the case of SO2 and NO2, the O atoms favor to stay at the top or bridge of Sc. Actually, the O atom from different molecules shares the same character in all cases. The NH3 shows the strongest adsorption behavior with Eads of −0.39 eV and vertical distance along the z direction of 1.54 Å. As shown in Figure , the NH3 molecule favors to stack parallel on the 2D sheets, with the N atom staying at the top of Sc and three H atoms sharing symmetrically three hollow sites of neighboring F atoms. The coupling between the nanosheet and NH3 will change the bond angles of H–N–H from 106.57 to 107.27°; meanwhile, the F atoms of nanosheets near the adsorption site move slightly away from the original position because the N partially shares the lone pair of electrons with Sc.

Figure 1

Most stable adsorption configurations for 11 molecules (light gray balls for Sc atoms, gray for C, light blue for F, white for H, dark blue for N, red for O, and yellow for S). The upper panels are the top view, and the lower panels are the side view of each molecule.

Table 1

Adsorption Energy per Unit Cell (Eads), Vertical Distances along z Direction (DZ), Average Distance to the Parent Nanosheet (D), Charge Transfer (CT), Bandgaps (Eg), Electron (me) and Hole (mh) Effective Mass, and Work Function of Molecules Adsorbed on the Surface of Sc2CF2a

	Eads (eV)	DZ (Å)	D (Å)	CT (10–3e)	Eg (eV)	me* (m0)	mh* (m0)	WF (eV)
bare					1.02	0.98	2.32	4.71
NH3	–0.39	1.54	2.53	–83.5	0.99	0.71	2.03	4.57
NO2	–0.21	2.33	2.93	100.3	0.99	0.90	2.13	5.17
SO2	–0.21	2.59	3.02	8.9	1.00	0.69	1.93	4.72
H2O	–0.20	1.81	2.24	–19.7	0.99	0.68	1.96	4.99
H2S	–0.18	2.28	2.38	–4.7	1.00	0.89	1.91	4.99
CO2	–0.16	2.57	3.04	4.6	1.01	0.90	2.18	4.73
CO	–0.14	2.66	3.13	–2.7	0.99	0.79	1.97	4.86
NO	–0.13	2.92	3.38	1.2	1.00	0.90	2.13	4.44
O2	–0.09	2.64	3.21	–1.7	0.99	0.81	1.92	4.99
H2	–0.07	2.28	2.64	–3.9	1.00	0.68	1.96	4.84
N2	–0.09	3.19	3.58	0.5	1.00	0.93	2.08	4.75

Negative CT values represent electron transfer from molecules to the Sc2CF2 sheet.

Once the molecule is adsorbed on the surface, the electronic structures of whole systems change and charge will redistribute between the molecule and nanosheet. Bader charge calculations are performed to determine the electron on each single atom so that we can see the charge transfer quantity induced by the gas molecule adsorption. As shown in Table and Figure a, small amount of charge transfer (0.0005–0.0046 e) has been found in diatomic molecules and non-polar cases (N2, H2, O2, CO, NO, and CO2), and more charge transfer can be found in polar polyatomic molecules. The H2S, H2O, and NH3 act as electron donors and transfer 0.0047, 0.0197, and 0.0835 e to Sc2CF2, respectively, while SO2 serves as an electron acceptor and obtains 0.0087 e from Sc2CF2. Especially, the largest charge transfer happens at the NO2 system, and NO2 takes 0.1003 electron from 2D materials, although the vertical distance between NO2 and Sc2CF2 is larger than that of the NH3 system. Furthermore, we calculated the charge density difference and projected it along the z direction as shown in Figure b. It can be seen that the polar polyatomic molecules’ adsorption systems show obvious oscillation at the interface region, especially the NH3, NO2, and H2O systems, which is consistent with the result from Bader analysis. To show the details, the spatial charge density difference for NO2 and NH3 adsorption on the Sc2CF2 surface is plotted in Figure c,d, respectively. In the case of NO2, all atoms at NO2 and even the O atoms, which correspond with the three immediate neighboring positive peaks at about 13.5, 14, and 14.5 Å in Figure b, will accept electrons from the substrate and even from Sc atoms, which are located far away from the interface (see Figure c). The adsorption of NO2 will induce a large interface dipole at the interface region, with electron accumulation at the NO2 molecule side and electron depletion at the substrate side. As NH3 attaches on Sc2CF2, electrons from the NH3 molecule orbital, which are mainly from the three H atoms in Figure d and correspond with the large valley near 14 Å in Figure b, transfer into Sc2CF2 and mainly into F atoms at the interface (see Figure d). Meanwhile, the adsorption of NH3 will induce a surface dipole at Sc2CF2, with electron accumulation near the surface of F atoms and electron depletion near Sc atoms at the subsurface. In the case of H2O, H atoms still prefer to donate electrons, which correspond the valley near 14.5 Å, and O atoms still like to accept electrons, which correspond the peak near 13 Å in Figure b. In total, few electrons will transfer from H2O into the substrate, which also produces a small surface dipole at the surface of Sc2CF2. According to the transferred charge quantity, we can see the selectivity of Sc2CF2 to detect the NO2 molecule.

Figure 2

(a) Charge transfer from molecules to Sc2CF2 (calculated from Bader charge analysis). (b) Planar average charge density difference projected along the z direction. Spatial charge density differences of (c) NO2 and (d) NH3 adsorbed on Sc2CF2 with an isosurface of 0.05 e/Bohr3. The purple and magenta regions indicate electron accumulation and depletion, respectively. The arrow specifies the direction of electron transfer.

To explain the physical causes of charge transfer, we calculated the band structures and density of states (DOS) of the adsorption system and extracted individual contributions of the molecule and substrate. Figure show the results for NH3, SO2, NO, and NO2 adsorption systems. Several flat bands appear after adsorption, whose position has a close relationship with the alignment of frontier molecular orbitals of molecules and bands of pristine Sc2CF2. The isolated NH3 molecule has a HOMO–LUMO gap of 5.4 eV from DFT calculation. Both the HOMO and LUMO of isolated NH3 are far away from the VBM and conduction band minimum (CBM) of pristine Sc2CF2; thus, the flat bands of the NH3 adsorption system stay away from the Fermi level. However, the shortest vertical distance between NH3 and Sc2CF2 makes the strongest wave-function mixing or electronic state coupling, which induces the large peaks at the energy range from −2.5 to −0.3 eV at the partial DOS plot in Figure a. The strongest wave-function coupling opens a charge transfer channel, which results in a large amount of charge transfer between NH3 and Sc2CF2. For the SO2 adsorption system, the LUMO of the molecule is located at the forbidden band region of Sc2CF2, while the HOMO is located at the deep valance band region as seen from Figure c. The vertical distance is longer than that of the NH3 system; thus, there is less wave-function mixing, demonstrating much less charge transfer. For open-cell NO, the half-filled doubly degenerated antibonding HOMO (2π orbital with spin-up), together with its spin-split partner unoccupied LUMO (2π orbital with spin-down), becomes non-degenerated because of the symmetry breaking after contacting with the substrate. We named the non-degenerated HOMO orbital at the higher energy as HOMO1 and the other as HOMO2, and the non-degenerated LUMO at the lower energy level as LUMO1 and the other as LUMO2 for convenience. The two HOMO orbitals are mid-gap states after adsorption, and the initial empty HOMO1 stays above the Fermi level. As a polar hetero-nuclear diatomic molecule, the Eads of NO is pretty small, which means weak wave-function mixing and is consistent with the small amount of charge transfer from the Bader calculation. It should be noticed that HOMO1 is still contributed from electrons of spin-up while HOMO2 is contributed from electrons of spin-down, which is totally different from the isolated NO molecule, indicating the large decrease of magnetic moment after adsorption (from 1 to 0.06 μB). The case of the NO2 adsorption system is very different from the three cases above, where the LUMO (6a1 molecule arbitral with spin-down) of NO2 is located below the VBM of Sc2CF2; thus, a large amount charge will transfer from the substrate to NO2 to fill the LUMO orbital of NO2. The valance band at some K points of the NO2 adsorption system is partially occupied or even unoccupied, and metal behaviors indicate a total change of the electronic structure after NO2 adsorption. Besides, the charge transfer between the Sc2CF2 substrate and paramagnetic NO2 also reduces the magnetic moment of the whole system by 0.91 μB.

Figure 3

Band structures and projected DOS of (a) NH3, (b) NO2, (c) SO2, and (d) NO adsorption systems. In the left panels, the gray lines are the band structure of the whole system, and the Fermi level is set as 0. The blue spheres correspond to the states with valid contribution from molecules, and the radii of spheres are proportional to the weight. In the middle and right panels, the dark gray lines filled with gray solid pattern are the total DOS of systems, and the blue lines are the PDOS of the molecules.

The rest electronic structure information of the molecules is summarized in Table . The bandgap after adsorption does not change much according to Table ; however, the effective mass changes because of the wave-function mixing. For instance, the orbital of SO2 couples more strongly with the conduction band of Sc2CF2 (Figure c), and the effective mass of the hole changes by 0.39 m0. Besides, the effective mass of both electrons and holes decreases for each adsorption case, which means the increasing of carrier mobility. It is worth noting that the work function after NO2 adsorption increases by 0.46 eV, which is even larger than what has been reported for graphene.[58]

Furthermore, we explored the optical properties for each system, which may be useful in optical chemistry gas sensors. The imaginary part of dielectric function and adsorption coefficient are plotted at Figure . At the range of visible light, little change can be found except for a slight intensity decrease and negligible red-shift of the main peaks. However, a clear difference occurs at the ultraviolet range. There are two major absorption peaks (located at 5.3 and 9.4 eV) at the ultraviolet range for pristine Sc2CF2. The intensity of the peak located around 5.3 eV is greatly enhanced after molecules adsorbed at each adsorption system, especially for the case of NO2. Meanwhile, an evident blue-shift happens, and the corresponding peak at the NO2 system moves into around 5.7 eV. The other peak around 9.4 eV changes little except for a slight red-shift. More importantly, the adsorption of each molecule induces a new peak around 9.9 eV, and the NO2 system shows the strongest absorption intensity here. It should also be noted that the adsorption of NO2 induces two additional peaks, which are located around 0.1 and 7.0 eV, respectively. The peak of 0.1 eV has a direct relationship with the partial occupied or unoccupied states near VBM. The quite different optical properties after NO2 adsorption indicate the potential application of Sc2CF2 as a NO2 optical chemistry sensor.

Figure 4

Imaginary part of dielectric function for the polarization vector perpendicular to the surface and absorption coefficient of Sc2CF2 systems with and without molecules adsorbed on the surface.

In the aspect of electronic and optical structures, we have found the selectivity of Sc2CF2 to detect NO2. To explore the adsorption thermal stability of NO2 at the Sc2CF2 surface, we performed ab initio molecular dynamics (MD) simulations with an NVT ensemble at a temperature of 300 K for about 11 ps. The time evolution of vertical distances between each atom of NO2 and the top F atom layer of Sc2CF2 is summarized in Figure a, in which two O atoms of NO2 are labeled as O-1 and O-2 to distinguish them. During the MD process, the Sc2CF2 layer prefers to contact with O atoms (O-1 or O-2) of the NO2 molecule, and the smallest vertical distance is maintained at the range of 2–4 Å. Compared with the initial structure, the system changes most at time 5.1 ps, when the vertical distance of N, O-1, and O-2 reaches up to 4.35, 5.49, and 3.72 Å, respectively. However, the NO2 molecule bounces back rapidly, and the vertical distance of N, O-1, and O-2 becomes 3.18, 3.11, and 2.33 Å at time 5.7 ps, respectively. From the MD simulation, it can be seen that the NO2 molecule only vibrates around its equilibrium positions, revealing that NO2 can adsorb on the 2D surface stably at room temperature.

Figure 5

(a) Evolution of vertical distances for each atom of NO2 with the top F layer of Sc2CF2 at first-principles molecule dynamics simulations. Snapshot of the NO2 adsorption system at time of (b) 0 ps, (c) 5.1 ps, (d) 6.9 ps, and (e) 11.1 ps, respectively.

Strain modulation is reported as an effective method to improve the performance of 2D materials. A uniaxial strain can be applied through bending, rolling up, and elongation in experiments, and a homogeneous biaxial strain can also been applied into 2D materials through a substrate that has a different thermal expansion coefficient with 2D materials or a substrate that is a piezoelectric material.[59−62] Most 2D materials have a fracture strain value over or about 10%,[63−66] and we enlarge the lattice of the Sc2CF2 substrate by 1–9% to explore the strain effect on adsorption energy and sensitivity. As shown in Figure , the adsorption of NO2, NH3, and H2O changes the most, and other systems are not sensitive to biaxial strain. As biaxial strain increases, the Eads’s of NH3 and H2O increase monotonously. For the strained NO2 system, the Eads’s start to increase with strain up to 4%, and the increase rate is much larger than that of both NH3 and H2O. As we all know, the adsorption energy can be influenced by the polarity of the molecules at gas molecule adsorption systems, and the polar molecule usually shows a larger adsorption energy than the nonpolar molecules as we discussed above. Besides, the atoms at the surface and subsurface should also have an effect on the adsorption energy. For instance, the well-saturated surfaces usually prefer to adsorb no molecule or adsorb molecules weakly, while the surfaces with unsaturated bonds usually like to adsorb molecules more strongly. When biaxial strain is applied, the symmetry of Sc2CF2 is destroyed to some degree and becomes not so well-saturated, and the adsorption of molecules will be enhanced. Thus, strain modulation is an effective method to improve the adsorption energy and sensitivity. We also explored the effect of biaxial strain on charge transfer, as shown in Figure . Different from the adsorption energy, the charge transfer is not sensitive to strain even for the NO2 system with strain less than 3%, and it seems that the charge transfer has no direct relationship with the adsorption energy for each molecule adsorption system. The charge transfer is on one hand affected by the distance between them, and a smaller distance usually results in a large charge transfer. On the other hand, the electron state coupling will also make a great contribution, and the molecular orbitals of gas molecules, which have a similar energy level with the electron states of Sc2CF2, are the charge transfer channel. The biaxial strain can hardly change the molecular orbitals; thus, we can see that the charge transfer is not sensitive to biaxial strain for most systems. However, in the case of NO2 adsorption, the LUMO of NO2 is very close with the VBM of Sc2CF2. To demonstrate the effect of strain on charge transfer in the case of NO2, we further explored the band structures of the NO2 adsorption system with a biaxial strain of 1, 4, and 7%, as shown in Figure S2 of the Supporting Information. It can be seen that the band structures, mainly the band (molecule orbital) alignment, are modified much with biaxial strain. When there is no strain, the alignment is type III, with the LUMO of NO2 below the VBM of Sc2CF2. The alignment becomes type II with the LUMO of NO2 moving up above the VBM of Sc2CF2 once strain is 1% at Sc2CF2. The band structure of 4% biaxial strain is similar with that of 1%; however, the band structure is altered much when strain continues increasing. When strain is 7%, the alignment reverts into type III with the LUMO of NO2 below the VBM of Sc2CF2, which is due to the large biaxial strain breaking the lattice symmetry of the Sc2CF2 substrate and thus modifying the electronic structure of Sc2CF2 near the band edge. In total, biaxial strain can enhance both the adsorption energy and charge transfer for the NO2 adsorption system and is an effective method to improve the sensitivity and selectivity of Sc2CF2 to detect the NO2 molecule.

Figure 6

The transferred charge and adsorption energy of each system when applied with biaxial strain from 1 to 9%.

Under real situations, 2D materials are typically supported by a substrate, which influences their electronic properties as well as the adsorption of molecules on their surface. SiO2 is a common substrate for 2D material applications in electronics or optoelectronics, which is cheap, flat, and chemically inert. However, SiO2 really can give a great effect on the properties of 2D materials, and some researchers try to use h-BN as a substrate or use h-BN as a substrate on top of SiO2/Si, which will reduce the effect of SiO2.[67,68] Here, we considered h-BN as a substrate for Sc2CF2 and explored the effect of h-BN. The lattice constant of h-BN after optimization is 2.41 Å. We used a 3 × 3 Sc2CF2 supercell to match with 4 × 4 h-BN vertically as shown in Figure a, and the mismatch is about 1.43%. After optimization, Sc2CF2 is still flat and there exists some buckling in h-BN, which however still maintains the honeycomb structure from the top view. It can be seen from the projected band structure of Sc2CF2/h-BN heterostructures in Figure b that the electronic structure of Sc2CF2 can be hardly affected by the h-BN substrate, and a type III band alignment is found. Then we considered four polar polyatomic molecules, that is, NO2, NH3, SO2, and H2O, adsorbed at the surface of Sc2CF2/h-BN. The calculated Eads and charge transfer from Bader charge analysis are also plotted in Figure c. Here, we used a 3 × 3 supercell, which is a little different from the model before and may have an effect on the value of Eads and charge transfer. However, the trends of both the adsorption energy and charge transfer are not modulated. The NH3 shows the largest adsorption energy, and the Eads of NO2, NH3, and SO2 is about 0.21 eV. The largest charge transfer occurs in the case of NO2, and NH3 shows more charge transfer than both H2O and SO2. Furthermore, we calculated the projected band structures for four adsorption systems with the h-BN substrate as shown in Figure S3 in the Supporting Information. In total, the relative position of the molecular orbital with bands of Sc2CF2 changes little, and the LUMO of NO2 is still partially occupied after absorption, which induces a large amount of charge transfer. These results demonstrate that the h-BN substrate can hardly affect the main results from calculations with a free-standing Sc2CF2. Also note that we considered a pristine Sc2CF2 layer here and the charge transfer will always enhance the carrier density. However, the defect is inevitable in experiments. If the presence of defects makes Sc2CF2 show n-type/p-type conductivity, the carrier density and conductivity do not depend on the charge transfer only. The electron/hole transfer into Sc2CF2 will improve/decrease the n-type conductivity, and the hole/electron transfer into Sc2CF2 will improve/decrease the p-type conductivity. Thus, the predicted obvious selectivity and sensitivity of Sc2CF2 to detect NO2 still need to be confirmed by future experiments.

Figure 7

(a) Atomic structures of the Sc2CF2/h-BN supercell: top view (upper) and side view (lower); pink balls are B atoms, and blue balls are N atoms. (b) Projected band structure of the Sc2CF2/h-BN supercell: gray parts are the contribution from Sc2CF2, and orange parts are the contribution from h-BN. (c) Charge transfer and adsorption energy of NO2, NH3, SO2, and H2O adsorbed on the surface of Sc2CF2 with h-BN as a substrate.

Conclusions

In summary, we have studied several molecules’ adsorption behaviors on Sc2CF2 and explored the charge transfer between molecules and Sc2CF2. Using first-principles calculations, we have investigated the structural, electronic, and optical properties of adsorption systems. The results show that the largest charge transfer quantity happens between NO2 and Sc2CF2, and optical properties of the system are modified by the NO2 molecule obviously, both of which are due to the special electronic structure, where the LUMO of NO2 is below the VBM of Sc2CF2. Thus, Sc2CF2 can detect the NO2 molecule selectively. Moreover, the MD simulation shows that NO2 can adsorb on the Sc2CF2 surface stably at room temperature. Besides, we explored the effect of biaxial strain on the Eads and charge transfer quantity of each system, and the result shows that the biaxial strain can enhance both adsorption energy and charge transfer and improve the sensitivity of Sc2CF2 to detect the NO2 molecule. Furthermore, we investigated how h-BN will affect the adsorption behavior and charge transfer, revealing that the h-BN is a potential substrate for Sc2CF2 sensors. Our result predicts that Sc2CF2 can be a promising selective and sensitive sensor to detect the NO2 molecule.

Computational Details

All first-principles calculations were carried out using the Vienna Ab initio Simulation Package (VASP)[54] based on DFT with a plane-wave basis set. To treat the exchange–correlation interaction of electrons, we chose the Perdew–Burke–Ernzerhof (PBE) functional[55] within the generalized gradient approximation (GGA). The electron–ion interactions were described by the projector augmented wave (PAW) potentials.[56] The energy cutoff for the plane-wave basis was chosen as 600 eV, and the Brillouin zones (BZs) were sampled by k grids with a uniform spacing of 2π × 0.02 Å–1. Convergence criteria of 10–4 eV for total energy and 0.01 eV/Å for force were adopted for self-consistent calculation and geometry optimization, respectively. We chose a 4 × 4 supercell to simulate the periodic structure of Sc2CF2 monolayers, and a vacuum spacing of 20 Å was added along the direction perpendicular to the 2D sheet (z direction) to avoid interaction between adjacent layers. To properly describe the van der Waals interaction between gas molecules and the Sc2CF2 sheet, we adopted the semi-empirical dispersion-corrected DFT-D3 scheme proposed by Grimme.[57] Spin polarization was included for the systems adsorbed by paramagnetic molecules like NO, NO2, and O2. Dipole correction was added in the average potential calculation for the system adsorbed by polar molecules. Various initial configurations with different adsorption sites and molecule orientations of the gas molecules were considered here. The initial distance between the gas molecule and 2D nanosheet was chosen to be 2.0 Å.