The discovery of graphene is followed by several other novel 2D nanosheets, like boron nitride, silicene, antimonene, arsenene, germenene, etc . These 2D sheets are widely been explored for having interesting electronic, optical, and transport properties [2-9]. At the same time, realizing the extra degree of chemical and physical freedom provided by the nanosurfaces, several groups have explored different molecules’ sensitivity towards it both theoretically and experimentally [10-14]. In the race to find a novel 2D-monolayer, recently, Al2O3 nanosheet is predicted and its stability has been confirmed theoretically . Yet the finding is in very infant mode, experimental establishment of the monolayer needs to be investigated. On the other hand, eradicating poisonous gases from the environment is a need of the hour. To accomplish this aim, the scientific communities have worked on a war footing [16-19]. NH3 and PH3 are toxic hazardous molecules that cause severe health effects in the human body. The NH3 is released into the environment during the production of pesticides, plastics, explosives, dyes, and tyles. The NH3 harms human bodies by interfering with the normal function of the eyes, throat, and skin. It also consequently decreases the concentration level of oxygen in hemoglobin, hence long-time exposure could be the reason for metabolic activities break down [20-21]. Phosphine in the semiconductor industry works as an agent to introduce phosphorus into silicon crystals. PH3 is used as an insecticide and pesticide for pollen grains . But its excessive presence in the environment leads to abnormalities in human development. PH3 attacks the central nervous system and lungs which leads to death. The symptom of PH3 exposure to human health could be nausea, vomiting, abdominal pain, diarrhea, chest pain, etc. The collective observation of the novelty of the Al2O3 nanosheet and the demand for poisonous gas sensors, motivate us to explore the sensitivity of Al2O3 monolayer towards the ammonia (NH3) and phosphine (PH3) molecules using quantum chemical investigation based on density functional theory (DFT). Previous works have shown dangling lone pairs from NH3 and PH3 played an important role in their binding with different nanosurfaces [23, 24]. Thus, any electron-deficient region on the Al2O3 nanosurface could be the NH3/PH3 catcher.
A 2x2 supercell consisting of 20 atoms (10-Al, 10-O) is employed for ground state calculations. Along the z-axis, a large lattice vector of 20Å is chosen to avoid mirror image interaction. After getting ground state geometry, NH3, and PH3, respectively, are brought near the 2D surface, and allow the configuration to get relaxed within the DFT-based self-consistent field. Interaction with the surface is further explored by investigating structural and electronic changes. All calculations have been performed in the framework of density functional theory (DFT) using Troullier Martins’s norm-conserving pseudo-potential . The valence electrons are defined with double zeta double polarized (DZDP) basis set. The exchange-correlation part of hamiltonian has opted for generalized gradient approximation (GGA) in the form of Perdew, Burke, and Ernzerhof (PBE) . An energy cutoff of 250Ry in the real-space grid is employed for numerical integration, and the choice for sampling k-point in the Brillouin zone is 9x9x1 using Monkhorst–Pack scheme . The conjugate gradient algorithm has been used for lattice vectors and atomic position optimization. The Spanish initiative for electronic simulations with thousands of atoms (SIESTA) quantum chemical package  is used for all geometrical and structural investigations.
STRUCTURAL AND ELECTRONIC PROPERTIES
The optimized geometry of Al2O3 is displayed in Fig.1, the in-planner atomic configuration has an average Al-N bond length of 1.69eV as reported before . In the previous report, the stability of the 2D sheet (or monolayer) is predicted through phonon calculation. Therefore, we are confirmed with the ground state stability of the obtained optimized geometry in the present calculation. The pristine Al2O3 nanosheet has been further exploited to examine the adsorption of NH3 and PH3 on its surface, respectively. Al2O3 crystal has an ionic Al-O bonding with Al and O as electron-deficient and electron-rich centers, respectively. We are anticipating a similar bonding pattern in our calculation, hence, expect the electron-rich center of NH3 and PH3 to interact with the electron-deficient site (Al) as stated above. The present investigation reveals NH3 and PH3 interact with the surface of the Al2O3-nanosheet through distances of 2.12Å and 2.51Å, respectively. The toxic molecules are directed towards the 2D surface with lone pairs on respective pentavalent atoms (N and P). The closest atoms between the two fragments within NH3-Al2O3 and PH3-Al2O3 configuration are N, Al, P, and Al, respectively, as shown in Fig. 2a and 2b. Different orientations of the XH3(X = N or P) molecule were relaxed on the Al2O3 monolayer and subsequent findings through formation energy calculation favored the depicted configuration of Fig. 2. Formation energy for NH3/PH3-Al2O3 is evaluated as
, and are the total energy of XH3-Al2O3, Al2O3-nanosheet and XH3 molecule, respectively (where, X = N or P). The respective interactions of NH3 and PH3 are -1.41eV and -0.70eV, suggesting strong interaction with the monoatomic layer. With these findings, we are confident that a substantial change in an electronic configuration has taken place. This will be explored next through the partial density of state (PDOS) and band structure analysis.
Energy band structure analysis suggests the Al2O3 monolayer has an energy bandgap of 4.45eV (Fig. 3a), hence falling in the range of wide bandgap semiconductors. It is a direct bandgap semiconductor with conduction band minima (CBM) and valence band maxima (VBM) lying at Г-point in k-space. A closed look at the energy band dispersion of the Al2O3-sheet reveals the flattening of energy band spectra after NH3/ PH3 interaction (Fig. 4 a) and (b). As band curvature is inversely proportional to the effective mass of the charge carrier, we expect the charge carrier (electron/hole) mobility of the 2D sheet will become sluggish after NH3/ PH3 adsorption. A slight reduction in the forbidden gap is observed for the molecularly adsorbed 2D sheets. A bandgap of 4.19eV ad 4.24eV is witnessed for NH3-Al2O3 and PH3-Al2O3 sheets, respectively. The holding of NH3/PH3 over the surface has further been confirmed through electronic charge transfer analysis. Mulliken population analysis identified a 0.42e charge transferred from NH3 to the Al2O3 sheet. The charge contribution is mainly attributed to the electronic charge transfer from the lone pair of N to the p-orbitals of Al. A similar pattern is observed for PH3-Al2O3 configurations with an electronic charge transfer of 0.33e. In Table.1, we have depicted important electronic and structural parameters to scrutinize the NH3/ PH3-Al2O3 monolayers. The role of inter-frontier orbital is very important in deciding the chemical behavior of an atomic configuration. In the present investigation, the highest occupied molecular orbital (HOMO) consists of the Al-3p state and the lowest unoccupied molecular orbital (LUMO) is derived from hybridized Al-3p and O-2p state, as evident from the contour plot (Fig. 4). The same is demonstrated in the PDOS plot of pristine Al2O3 (Fig. 3b), no state available is close to the Fermi level. This was also confirmed through band-dispersion analysis in a band structure study. To predict the interaction taking place between NH3/PH3 and the Al2O3-sheet, fragmented DOS is shown in Fig. 5 (b) and (d), respectively. The availability of NH3/PH3 and the Al2O3-DOS at the inter-frontier orbital region (Fig. 3b) and (3d) suggests robust hybridization between molecular orbital from NH3/PH3 and the Al2O3 sheet. The N/P-pz orbital gets hybridized with the pz orbital of Al which causes it to hold NH3 and PH3 over the 2D surface. The conductivity of Al2O3 upon interaction with NH3/PH3 could be computed using the below equation
where K is the proportionality constant, Eg defines the energy bandgap, KB is the Boltzmann constant and T is room temperature (300K).
As discussed above, NH3/PH3 interaction Eg decreased which signifies a rise in electrical conductivity. However, energy band flattening in the band structure diagram suggests increased effective mass of charge carriers, hence, decreased mobility. Thus, a competing condition is raised to determine the transport behavior of the NH3/PH3-Al2O3 nanosheet. The depiction of the electrostatic potential difference contour is shown in Fig. 3. The respective dark and light regions signify negative and positive charge accumulation. Relatively, more charge accumulation could be noticed between NH3 and the Al2O3 surface compared to the PH3. Although the differences are very feeble as mentioned in Mulliken’s population analysis, the contour plot depicts a localized charge pattern at O-atoms indicating ionic-bonding character within Al2O3-monolayer similar to the Al2O3-matrix.
In this work, we investigated the interaction pattern of NH3/PH3 towards the Al2O3-monolayer. The structural and electronic findings suggest a strong affinity of the toxic molecules towards the 2D nanosheet. With a binding strength of -1.41eV and -0.70eV the molecules were chemisorbed on the 2D surface. The 2D surface holds NH3/ PH3 at a distance of 2.12Å/2.51Å with 0.42e/0.33e charges transferred from the latter to the former. The electronic bandgap of the Al2O3-monolayer gets reduced by 0.26eV and 0.21eV, respectively, on interaction with NH3 and PH3. Envisaging inclusion of the toxic molecular state in the inter-frontier orbital region of the Al2O3-nanosheet is confirmed in our findings through PDOS analysis. Strong hybridization of lone pair orbital from NH3/PH3 with the valence p-orbitals of O is the reason for electronic charge transfer between the two fragments. The conductivity equation and bandgap flattening indicate delicate competition in deciding charge transport characteristics of the NH3/PH3-Al2O3 configurations.
CONFLICT OF INTEREST
The authors declare no conflict of interest.