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Article

Molecular Adsorption of NH3 and NO2 on Zr and Hf Dichalcogenides (S, Se, Te) Monolayers: A Density Functional Theory Study

by
Shimeles Shumi Raya
,
Abu Saad Ansari
* and
Bonggeun Shong
*
Department of Chemical Engineering, Hongik University, Seoul 04066, Korea
*
Authors to whom correspondence should be addressed.
Nanomaterials 2020, 10(6), 1215; https://doi.org/10.3390/nano10061215
Submission received: 10 June 2020 / Revised: 18 June 2020 / Accepted: 20 June 2020 / Published: 22 June 2020
(This article belongs to the Special Issue Electronics, Electromagnetism and Applications of Nanomaterials)
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Abstract

:
Due to their atomic thicknesses and semiconducting properties, two-dimensional transition metal dichalcogenides (TMDCs) are gaining increasing research interest. Among them, Hf- and Zr-based TMDCs demonstrate the unique advantage that their oxides (HfO2 and ZrO2) are excellent dielectric materials. One possible method to precisely tune the material properties of two-dimensional atomically thin nanomaterials is to adsorb molecules on their surfaces as non-bonded dopants. In the present work, the molecular adsorption of NO2 and NH3 on the two-dimensional trigonal prismatic (1H) and octahedral (1T) phases of Hf and Zr dichalcogenides (S, Se, Te) is studied using dispersion-corrected periodic density functional theory (DFT) calculations. The adsorption configuration, energy, and charge-transfer properties during molecular adsorption are investigated. In addition, the effects of the molecular dopants (NH3 and NO2) on the electronic structure of the materials are studied. It was observed that the adsorbed NH3 donates electrons to the conduction band of the Hf (Zr) dichalcogenides, while NO2 receives electrons from the valance band. Furthermore, the NO2 dopant affects than NH3 significantly. The resulting band structure of the molecularly doped Zr and Hf dichalcogenides are modulated by the molecular adsorbates. This study explores, not only the properties of the two-dimensional 1H and 1T phases of Hf and Zr dichalcogenides (S, Se, Te), but also tunes their electronic properties by adsorbing non-bonded dopants.

Graphical Abstract

1. Introduction

Nanomaterials often manifest fascinating and useful properties, which can be exploited for a variety of applications [1,2,3,4,5,6,7,8,9]. For example, electronic devices are miniaturized to nanoscale. However, this development faces some issues, such as replacing the currently used SiO2 gate oxides of complementary metal–oxide–semiconductor (CMOS) transistors with another high-k material [10]. Also, for sub-10-nm field-effect transistors (FETs), effective gate control is needed. Furthermore, Si suffers from surface roughness (SR) effects that can reduce their charge carrier mobility [11] and lead to strong variability in threshold voltages [12]. Encouragingly, in recent years, the introduction of high-k gate dielectrics and metal gates has been successful for improving transistor performance [13]. However, the current International Technology Roadmap for Semiconductors (ITRS) predicts that, to fulfill the expected demand for nanodevices, novel materials with extreme properties will be needed to successfully address the challenges of transistor scaling in the next decade [14]. A current focus of nanotechnology is on atomically thin semiconductor materials. The use of two-dimensional (2D) materials enables nano-scale transistors without dangling bonds. However, new challenges exist, such as bandgap, non-negligible contact resistance, and the difficulty in integrating high-k gate insulators with most 2D materials. The fact that the large bandgap (Eg = 9 eV) of SiO2 and its high-quality interface with Si enables the isolation of Si components and a reduction of additional gate leakage currents is noteworthy. Thus, if one wants to replace Si in these materials, the candidate material must not only demonstrate properties similar to Si, but also, their native oxides should exhibit high dielectric constants.
Two-dimensional transition metal dichalcogenides (TMDCs) are gaining research interest due to their atomic thickness and unique mechanical, electric, and optical properties, further, they are considered as promising high-performance electronic and optoelectronic materials [15,16]. Depending on their chemical compositions and structural configurations, 2D TMDC materials can be categorized as metallic, semimetallic, semiconducting, insulating, or superconducting. A semimetal exhibits the feature whereby a small overlap exists between the top of the valance band and the bottom of the conduction band. For example, some group-IVB TMDCs show semimetal features due to a small overlap between the top of the p-orbital chalcogen valance band and the bottom of the d-orbital transition metal conduction band [17]. Many 2D TMDCs are semiconductors by nature, and possess a huge potential to be made into ultra-small and low-power transistors that are more efficient than state-of-the-art silicon-based transistors fighting to cope with ever-shrinking devices [16]. Semiconducting TMDCs have advantages over gapless graphene in applications for logic transistors, photodetectors, and FETs, since a sizable bandgap is necessary to achieve high on/off ratio, which these materials possess [16]. The most widely investigated semiconducting TMDC, MoS2, depicts good mobility (~100 cm2∙V−1∙s−1 in sub-2-nm-thick films) independent of channel thickness and a high on/off FET current ratio (~106) near room temperature [18,19]. Furthermore, MoS2 does not exhibit a large SR and is thus advantageous to be used in place of Si in sub-10-nm FETs.
Conversely, Zr- and Hf-based TMDCs demonstrate a moderate bandgap comparable to Si. Furthermore, they demonstrate the unique advantage that their native oxides (ZrO2 and HfO2) are excellent dielectric materials, which show potential to replace Si in semiconductor technology [20]. These TMDCs exhibit ohmic contact like Si with their native oxides, which enable the isolation of components, and they demonstrate a reduced leakage current compared to Si transistors. Although, Mo- and W-based TMDCs and their native oxides (MoO3 and WO3) depict similar features, MoO3 and WO3 are not good insulators, and they may even act as dopants [21,22,23].
The electronic and optoelectronic properties of present TMDC materials are sometimes not good enough, and additional candidate TMDC materials are being sought. So far, 2D semiconducting Zr- and Hf-based TMDCs from group IVB were not investigated as much as their counterparts from group VIB. Further, changing the chalcogen species (S, Se, Te) in TMDCs can trigger paradigm changes to their electronic structure, and in turn alter their electronic and optoelectronic attributes. Recently, Zr- and Hf-based TMDCs were theoretically predicted to exhibit higher mobilities and higher sheet current densities than group-VIB (Mo and W) TMDCs [14,24]. Inspired by this, 2D HfS2, HfSe2, and ZrS2 were studied for their potential applications in FETs and phototransistors [20,25,26]. However, further investigations are needed to shed light on 2D Zr- and Hf-based TMDCs for their potential applications, and new findings in nanoscience are subsequently anticipated.
In addition to implementing 2D materials in nanodevices, tuning of the material properties of 2D materials is very important. One possible method to precisely tune the material properties of 2D atomically thin nanomaterials is to adsorb molecules on their surfaces as non-bonded dopants [27]. Researchers demonstrated that the molecular adsorption of NO2, NH3, H2O, CO, borazine, triazine, and benzene on gapless graphene led to the band gap widening due to the adsorption-site-dependent magnitude of the band gap [28,29]. Such phenomenon is seen in the present investigation after NH3 adsorption. The molecular adsorption of NO2 and NH3 on 2D MoS2 was also studied by Luo et al. [30]. Adsorbing molecules demonstrate the potential to modify the electronic properties, which could be relevant for ultra-small low-power electronic devices. The adsorbing molecules serve as either an electron donor or acceptor, thereby producing a temporary charge perturbation in the adsorbing material. To date, no such study on 2D Zr- and Hf-based TMDCs was conducted.
In the present work, the molecular adsorption of NH3 and NO2 on 2D Zr and Hf dichalcogenides (S, Se, Te) are studied using density functional theory (DFT) calculations. The adsorption configuration, energy, and charge-transfer properties during molecular adsorption are calculated. In addition, the effects of the molecular dopants (NH3 and NO2) on the electronic structure of the materials are studied. Researchers observed that adsorbed NH3 donates electrons to the conduction band of the Zr (Hf) dichalcogenides, while NO2 received electrons from the valance band. The resulting band structure of the molecularly doped Zr and Hf dichalcogenides are modulated by the molecular adsorbates. Therefore, by introducing molecular dopants such as NH3 and NO2 to TMDCs, we confirm that the material properties of these substrates can be tuned.

2. Computational Methods

The DFT calculations, using slab models, were performed using Vienna ab initio simulation package (VASP) version 5.4.4 [31], employing Perdew-Burke-Ernzerhof (PBE) exchange−correlation functionals [32] and the projector-augmented wave method [33]. Dispersion interactions were considered via the Grimme method [34]. To optimize the lattice parameters of the metal dichalcogenides (MX2, M = Zr, Hf, and X = S, Se, Te), their unit cell structures were fully relaxed using the conjugate gradient method [35] until the maximum Hellmann–Feynman force acting on each atom was less than 0.02 eV∙A−1. The optimized lattice parameters are close to the reported values (Table 1), which, in later sections, will be discussed in detail. 4 × 4 × 1 trigonal prismatic (1H) and octahedral (1T) supercells containing 48 atoms were constructed using the calculated lattice parameters, which were employed to simulate the pristine 2D Zr (Hf) dichalcogenides (S, Se, Te). A spacing of 20 Å in the vertical direction was added to minimize any unphysical interactions between the slabs. A (5 × 5 × 1) gamma (γ) k-point mesh and an energy cutoff of 500 eV were used after testing the slab-energy convergence. Three different sites on the 1H and 1T supercell Zr (Hf) dichalcogenides, namely, TM (top of metal), TX (top of chalcogen), and TH (top of hexagon), were considered for NH3 (NO2) adsorption, as shown in Figure 1. Two orientations of the adsorbing gas molecules, i.e., the hydrogen (oxygen) atom of NH3 (NO2) oriented away from the adsorbent surface (U-orientation) or toward the surface (D-orientation) were considered. The adsorbates and the slab were allowed to relax until the residual force became less than 0.02 eV∙Å−1.
To characterize the interaction strength between the adsorbate gas molecule and the adsorbent material, the molecular adsorbates electronic binding energy (Ebinding) was obtained using the following equation [36],
E b i n d i n g = E t o t a l − E s u r f a c e − E a d s o r b a t e
where Etotal, Esurface, and Eadsorbate are the total electronic energies of the slab with adsorbates, the pristine Zr (Hf) dichalcogenides slab, and the free adsorbates, respectively. By means of a Bader analysis [37], the charge transfer between the monolayer substrate and the adsorbate was obtained. The adopted dispersion-corrected method is very accurate for structural and adsorption energy calculations. However, even though the bandgap values are underestimated, this method still yields reasonably correct features of electronic structure. Further, the charge density difference (Δρ) was defined to be consistent with previous calculations of a gas on a surface [35], which is calculated as ∆ ρ = ρ M X 2 + g a s − ( ρ M X 2 + ρ g a s ) , where ρ M X 2 + g a s , ρ M X 2 , and ρ g a s are the charge density of the gas-molecule-adsorbed TMDC surface, the pristine TMDC surface, and the isolated gas molecules, respectively.

3. Results and Discussion

Prior to NH3 (NO2) adsorption, the lattice parameters (a) of the 1T and 1H Zr (Hf) dichalcogenides were calculated. To the best of our knowledge, only the 1T structure has been taken into consideration. To date, no lattice parameter data, either theoretical or experimental, that are related to 1H Zr (Hf) dichalcogenides, were published and made available. Hence, in this work, the 1H structure lattice parameters are discussed by comparisons with the 1T structure. Table 1 summarizes the optimized lattice parameters (Å) of the 2D 1T and 1H Zr (Hf) dichalcogenides. The lattice parameters of 1H are slightly smaller in all cases, as compared to the 1T Zr (Hf) dichalcogenides, which are associated with a decrease in the ionic radius of S. This is due to differences in atom stacking: 1H possess an ABA-type atomic staking, while 1T demonstrates an ABC-type atomic staking. As the atomic indices change from Te to Se and from Se to S, the lattice constants and metal-to-chalcogen bond lengths decrease due to the decreased atomic radius of the chalcogen. When switching from 1T to 1H phases, similar changes occur. The maximum difference in the calculated lattice parameters of the 1H and 1 T structure is found for HfS2 (3.04%), whereas a minimum is found for ZrTe2 (0.77%); we find the overall order HfS2 > ZrS2 > HfSe2 > ZrSe2 > HfTe2 > ZrTe2. The calculated lattice parameters of the 1T-structured Zr (Hf) dichalcogenides are in close agreement with reported theoretical and experimental lattice parameters [38,39,40,41]. No experimental data are available for the lattice parameter of 1T-HfTe2 and 1T-ZrTe2. The variation of lattice parameters from previous theoretical data is due to the experimental conditions and the level of theory used for the calculations.
To investigate their electronic properties, we focused on the band structures of the 1T and 1H Zr (Hf) dichalcogenides and compared 1T relative to 1H in terms of their conduction bands, valance bands, and band gaps. Figure 2 depicts the band structures of the pristine 2D 1T and 1H Zr (Hf) dichalcogenides obtained by the PBE method. The band structures show that they are indirect band gap semiconductors, similar to corresponding bulk TMDCs. This feature is different to that seen for some other TMDCs, such as MoS2 or WS2. The bulk MoS2 and WS2 systems show indirect bandgap transitions, but they gradually shift to direct transitions for the monolayer [15]. Further, 1T-HfTe2 and 1T-ZrTe2 show semimetallic features instead of semiconductor features, which is consistent with earlier experimental [42] and theoretical [40,43] reports.
Table 2 summarizes the calculated band gaps of the pristine 2D 1T and 1H Zr (Hf) dichalcogenides (S, Se & Te) [39,40,41,43,44]. To date, no reports are available on the band structure and band gap of 1H Zr (Hf) dichalcogenides.
Researchers reported theoretically that in 1T-HfS2, the Hf–d and S–p states are located between −5 eV and the Fermi energy (EF) [40], whereas, the bands from EF to 2.4 eV (~ 3 eV for 1H-HfS2) consist of Hf–d and Hf–f states with a small contribution from the S–p state [40]. The valance band maximum (VBM) is observed at Γ, which is consistent with the calculations of Murray et al. [45], Mattheis [46], Fong et al. [47], and Reshak and Auluck [40]. The conduction band minimum (CBM) is located at M, in agreement with the calculations of Fong et al. [47] and Traving et al. [48]. Further, researchers reported theoretically that a strong hybridization exists between the Hf–f and Hf–d states below and above EF, respectively, and a weak hybridization exists between the Hf–d and S–p states below EF [40]. Conversely, the 1H-HfS2 band gap (1.15 eV) is slightly smaller than that of 1T-HfS2 (1.28 eV). The conduction band slightly shifts to a lower value of 0.15 eV. Further, we find that the energy band structures pattern of 1H-HfS2 is different to that of 1T-HfS2.
The band structure of 1T-HfSe2 is similar to that of 1T-HfS2, but it exhibits a smaller energy gap (Table 2). Replacing S with Se causes a separation of the Hf–f states from the Hf–d states below and above EF [40]. Further, the hybridization below EF between Hf–d and Se–p is stronger than in 1T-HfS2. Identical to 1T-HfS2, the VBM is located at Γ, and the CBM is at M, in agreement with Murray et al. [45]. Although, the band structures of 1H and 1T HfSe2 differ from each other, the positions of the valance and conduction bands are identical, and hence both show nearly the same band gap value.
In 1T-ZrS2, the VBM is located at Γ, while the CBM is located between Γ and K, resulting in an indirect gap of 1.13 eV. It is known theoretically that the band between −5 eV to EF and EF to 3 eV is composed of Zr-d and a small contribution from S-p states [43], as well as a strong hybridization between Zr-d and S-p states below EF [43]. The 1T-ZrS2 band looks similar to that of 1T-HfS2 but with a slightly decreased bandgap. Conversely, the 1H-ZrS2 bands not only match those of 1H-HfS2, but they also show nearly the same electronic gap. However, the 1H-ZrS2 band structure is quite different to that of 1T-ZrS2, although they demonstrate nearly the same bandgap (Table 2) with identical band positions.
In 1T-ZrSe2, the VBM is located at Γ, and the CBM is at M [43,45]. With some minor differences, the band structure of 1T-ZrSe2 is like that of 1T-ZrS2, such as the reduction in the bandwidth of the Se-S group that is shifted toward lower energies with respect to EF along with a second group band enhancement [43]. Furthermore, the shift in the conduction bands at about 0.5 eV toward lower energies leads to 1T-ZrSe2, demonstrating a smaller energy gap than 1T-ZrS2. In contrast, no shifting and enhancement of second group band toward EF in 1H-ZrSe2 than 1H-ZrS2. Additionally, the conduction band in 1T-ZrS2 shifts toward EF less than that of 1T-ZrSe2. Thus, 1H-ZrSe2 possesses a higher band gap (0.85 eV) than 1T-ZrSe2 (0.41 eV).
In 1T-ZrTe2 (1T-HfTe2), the occupied and unoccupied bands move toward EF, which closes the energy gap and indicates metallic behavior. Also, a strong hybridization exists between the Te–p and Zr (Hf) d states below EF [40,43]. Interestingly, 1H-ZrTe2 (1H-HfTe2) still shows semiconducting behavior, although its energy gap is lower than that of 1H-ZrSe2 (1H-HfSe2). A rise in the conduction band and fall in the valance band toward EF is clearly observed.
It is to be noted that the Eg value of ZrS2 (HfS2) is a maximum, and it decreases as we replace chalcogen S with first Se and then Te. Although the amount of decrease in the Eg value in the Zr dichalcogenides differs to that of the Hf dichalcogenides, their decreasing trends are similar. The band structure of the Zr dichalcogenides looks similar to that of the Hf dichalcogenides, except for the case of 1H-ZrSe2 and 1H-HfSe2 (where they look different), even though their Eg values differ. Furthermore, both 1T-HfTe2 and 1T-ZrTe2 depict semimetallic natures. In the 1H structure, as S is replaced by Se, a large reduction occurs in the Eg value for the Hf-based TMDCs relative to the Zr-based TMDCs, while the situation is reversed for materials containing 1T structures. Also, it is well-known that 1T-ZrTe2 (1T-HfTe2) possesses a semimetallic nature. However, in the case of 1H structures, both materials show band openings; 1H phase Zr(Hf) dichalcogenides depict a band gap energy ranging from 0.29–1.15 eV, while in the 1T phase, the range is 0.41–1.29 eV. These moderate Eg values are comparable to other semiconductor materials such as Si (Eg = 1.14 eV), Ge (Eg = 0.67 eV), and PbS/Se/Te (Eg = 0.37/0.27/0.29 eV) [49]. A semiconductor of this nature can lead to the use of these materials in different applications.
Thereafter, NH3 and NO2 adsorption on the trigonal prismatic (1H) and octahedral (1T) phases was carried out. First, we optimized the atomic geometries of the NH3 and NO2 gas molecules using the DFT approach. Based on our calculated results, the bond lengths of the NH bonds of the NH3 molecule and the NO bonds of the NO2 molecule are 1.02, and 1.21 Å, respectively. These are in reasonable agreement with previously reported data [50,51]. Moreover, the structure of the considered 4 × 4 × 1 supercell of Zr (Hf) dichalcogenides monolayer was geometrically optimized.
To search for the most stable configuration of the NH3 (NO2) molecules on the Zr (Hf) dichalcogenide monolayer, various adsorption positions were examined. For each adsorption configuration, an initial, reasonable distance between the gas molecules and the substrate was chosen. Figure 3 depicts the typical optimized geometry configurations of the NH3- and NO2-adsorbed 1H Zr (Hf) dichalcogenide monolayer. Configurations a–f represents the adsorption of NH3 molecules on the 1H Zr (Hf) dichalcogenides, while configurations g–l shows the interaction between the NO2 gas molecules and the 1H Zr (Hf) dichalcogenide monolayer. H-upward (U-orientation) is the preferable configuration for the NH3 molecules, except for the case of 1H-ZrS2, where H is oriented toward the adsorbing surface. Furthermore, the TM-site is preferred for NH3 adsorption on 1H-HfS2 and 1H-ZrSe2, where the adsorption-distances (d; i.e., the shortest atom-to-atom distance between the gas molecules and the substrate) are 2.43 and 2.49 Å (Table 3), respectively, suggesting NH3 adsorbs more strongly on 1H-HfS2 than 1H-ZrSe2. The remaining 1H Zr (Hf) dichalcogenides adsorb NH3 on the TH-sites with minimum adsorption-distances on HfSe2 (3.38 Å). Interestingly, for all cases, no NH3 adsorption takes place on the chalcogen sites. In contrast, NO2 preferably adsorbs only on TX-sites with D-orientation on HfTe2 and ZrTe2, with the same adsorption distance (2.28 Å). The U-oriented NO2 adsorbs on the remaining 1H Zr (Hf) dichalcogenides, with a minimum adsorption-distance of 2.02 Å on 1H-HfS2. Conversely, the NH3 (NO2) molecules adsorb differently on the 1T Zr (Hf) dichalcogenides.
The optimized geometric configurations of the molecularly adsorbed NH3 and NO2 on the 1T Zr (Hf) dichalcogenides monolayers are summarized in Figure 4, where configurations a–f and g–l represents the adsorption of NH3 and NO2 molecules on the 1T Zr (Hf) dichalcogenides, respectively. As compared to 1H, the U-orientation is favorable for NH3 on the 1T Zr (Hf) dichalcogenides except for 1T-ZrTe2, where the D-orientation is favorable. The largest adsorption-distance is 3.87 Å (Table 4) for NH3 on the 1T Zr (Hf) dichalcogenides. The D-orientation (in 1H) changes to U-orientation for NH3 on 1T-ZrS2. Additionally, the TM-site is preferred over the TH-site, which is the opposite situation compared with what was observed for the 1H Zr (Hf) dichalcogenides. Consistent with 1H, HfS2 shows a minimum NH3 adsorption-distance (2.42 Å). A fresh, no preference adsorption takes place on the chalcogen sites. Conversely, the D-orientation is favorable for NO2 on the 1T Zr (Hf) dichalcogenides, differing from the previous three cases, with a minimum adsorption-distance of 3.21 Å on 1T-ZrTe2, which is almost 1 Å larger than that found for 1H-ZrTe2. Furthermore, the TM and TX-sites are equally favorable. The details are given in Table 4. It is distinctly seen that in all four cases, the D-orientation exhibits a larger adsorption distance than the corresponding U-orientation. The smallest adsorption-distance on metal sites is found for NH3 on 1T-HfS2 (2.42 Å) and 1T-ZrS2(2.44 Å), which are larger than the sum of the covalent atomic radii of Hf-N (2.26 Å) and Zr-N (2.28 Å). Similarly, the smallest adsorption distance on the chalcogen is found for NO2 on 1H-HfS2 (2.02 Å), 1H-ZrSe2 (2.17 Å), and 1H-HfTe2(1H-ZrTe2) (2.28 Å), which are significantly larger than experimental average bond lengths of S-N (1.71 Å), Se-N (1.82 Å), and Te-N (2.02 Å) [52]. Thus, no chemical bonds are expected to form, and only the physisorption of NH3 and NO2 takes place on the 1H and 1T Zr (Hf) dichalcogenides.
A negative adsorption energy implies that the adsorption of the NH3 (NO2) molecules on the 1H and 1T Zr (Hf) dichalcogenides is energetically favorable [53]. The adsorption energy (Eads) of NH3 and NO2 on the 1H and 1T Zr (Hf) dichalcogenides at different sites (H-site, TM-site, TX-site) was calculated and plotted, as shown in Figure 5. In case of the 1H Zr (Hf) dichalcogenides, the largest calculated NH3 adsorption energy is −647 meV on 1H-HfS2, where adsorption took place on the TM-sites, and the smallest is −199 meV on the 1H-HfSe2 (TH-site). Researchers observed that the metal sites are more energetically favorable than the TH-sites, and no NH3 adsorption occurs on the TX-site. In addition, researchers observed that the U-orientation is preferred to the D-orientation on the TM-sites. Furthermore, 1H-HfS2, 1H-ZrS2, and 1H-ZrSe2 are more energetically favorable for NH3 adsorption than 1H-HfSe2, 1H-HfTe2, and 1H-ZrTe2. The order of favorability is HfS2 > ZrSe2 > ZrS2 > HfTe2 (ZrTe2) > HfSe2. Conversely, NO2 adsorbs only on the TX-sites of the 1H Zr (Hf) dichalcogenides. Compared to NH3, NO2 shows a high absorption favorability, as a more negative adsorption energy is observed on all surfaces (Table 3). Instead of 1H-HfS2, 1H-HfTe2 shows high favorability to NO2 adsorption. Interestingly, 1H-HfTe2 and 1H-ZrTe2 shows high NO2 adsorption energies, in contrast to the energies of NH3 adsorption. The order of NO2 adsorption favorability is HfTe2 > ZrTe2 > ZrS2 > ZrSe2 > HfS2 > HfSe2. Although NO2 shows high adsorption favorability relative to NH3 on the 1H Zr (Hf) dichalcogenides, NH3 is more favorable on 1H-HfS2 than NO2, which exhibits an adsorption energy almost 200 meV larger.
Table 4 depicts the calculated Eads values for NH3 and NO2 on the 1T Zr (Hf) dichalcogenides. NH3 shows an energetically high adsorption favorability on 1T-ZrS2 surfaces, with an Eads value of −587 meV (TM-sites) and a lower value on 1T-ZrTe2 (−166 meV, TH-sites) (Figure 5). The TM-sites show higher NH3 adsorption favorability than the TH-sites, consistent with what was seen for NH3 on the 1H Zr (Hf) dichalcogenides. The preferential adsorption site of NH3 shifts from the TH-sites to the TM-sites for 1T-ZrS2 and 1T-HfSe2. The order of NH3 adsorption favorability also changes, suggesting that the surfaces of the 1H and 1T Zr (Hf) dichalcogenides behave differently. Conversely, this behavior is consistent with the 1H surfaces of 1T-HfTe2 and 1T-ZrTe2, showing high NO2 adsorption favorability. Also, instead of only TX-sites, energetically favorable NO2 adsorption on TM-sites for 1T-HfSe2, 1T-ZrSe2, and 1T-HfTe2 is observed. Thus, NH3 adsorption is more energetically favorable than NO2 on 1T-HfS2, 1T-ZrS2 and 1T-ZrSe2, while the opposite situation exists for 1T-HfSe2, 1T-HfTe2, and 1T-ZrTe2.
It is noteworthy that, consistently with all cases, the 1H-phase demonstrates different Eads values with high adsorption favorability than the 1T-phase (Figure 5). This situation is reversed for NH3 on ZrS2, where the 1T-phase is more favorable for adsorption than the 1H-phase. Conversely, the values of Eads for HfSe2 and HfTe2 are almost the same in both phases.
Next, a Bader analysis was employed to estimate the charge (ΔQ) transfer from the NH3 (NO2) to the surface of the 1T-(1H-) Zr (Hf) dichalcogenides (or vice versa), where the change is either positive or negative. The charge transfer from the molecules to the dichalcogenide’s surface is defined as positive here, whereas from the surface to the molecules is negative. Researchers noticed that, in order for a charge transfer to occur, the property of the material, the adsorption sites on the dichalcogenide’s surface, and the orientation of the gas molecules are crucial factors [54].
Researchers found that the NH3 that adsorbs on the 1H Zr (Hf) dichalcogenides depicts a positive charge, suggesting NH3 acts as a charge-donor (Table 3), which is well known experimentally. Although NH3 strongly adsorbs on 1H-HfS2 (M-sites), the maximum charge transfer (0.152 e) takes place when NH3 adsorbs on 1H-ZrS2 (H-sites). Interestingly, the adsorption distance is a maximum in this case. The fact that NH3 donates more charges at M-sites than at H-sites on MoS2 is noteworthy [35]. The charges transferred to 1H-HfSe2, 1H-ZrSe2, 1H-HfTe2, and 1H-ZrTe2 are 0.045, 0.081, 0.033, and 0.009 e, respectively. Comparatively more charge transfer occurs on the Zr-based chalcogenides compared to the Hf-based chalcogenides when adsorbed on the M-sites, while the opposite situation is seen on the H-sites. It seems to be that D-oriented NH3 donates more charge than the U-oriented NH3. Conversely, NO2 accepts an electron from the 1H-Zr (Hf) dichalcogenides (Table 3). This behavior is expected as NO2 is a well-known charge acceptor [35]. Like NH3, NO2 also depicts a high charge transfer on 1H-ZrTe2 (0.622 e), where even 1H-HfTe2 demonstrates a high NO2 adsorption energy. Furthermore, higher charge transfers occur on the Zr-based chalcogenides than on the Hf-based chalcogenides, when adsorption occurs on the X-sites, considering the same chalcogen. Also, the D-oriented NO2 accepts more charge than the U-oriented NO2, except for the case of 1H-ZrSe2.
Consistent with what was observed for the 1H surfaces, the NH3 adsorbed by the 1T Zr (Hf) dichalcogenides also depicts a positive charge, implying that NH3 is a donor (Table 4). Comparatively, charge transfer enhances for the 1H-Zr (Hf) dichalcogenides, except for 1T-ZrS2 and 1T-ZrSe2, where it decreases. Interestingly, more charge transfer occurs on the Hf-based chalcogenides compared to on the Zr-based chalcogenides, due to the NH3 adsorption taking place on either H-sites or M-sites, considering the same chalcogen. Also, the D-orientation shows a minimum charge transfer of 0.018 e on 1T-ZrTe2, and it depicts a maximum for 1H-ZrS2. The maximum charge transfer occurs for NH3 adsorption on 1T-HfS2, while it is a minimum on 1H-HfS2, suggesting NH3 behaves differently on 1T surfaces than 1H surfaces. Conversely, NO2 accepts a maximum charge on 1T-ZrTe2, similar to 1H-ZrTe2. Furthermore, charge transfer is different on the 1T and 1H surfaces. Overall, NH3 donates charges in the range of 0.01–0.15 e on 1H and 0.02–0.2 e on the 1T Zr(Hf) dichalcogenides. In contrast, NO2 accepts charge in the interval of 0.18–0.62 e on 1H and 0.05–0.62 e on the 1T Zr(Hf) dichalcogenides.
Figure 6 depicts the correlation between Eads and ΔQ for NH3 (NO2) on the Zr (Hf) dichalcogenides. The NH3 (NO2) molecules on the Zr (Hf) dichalcogenides depict a direct correlation between Eads and ΔQ, where a low Eads results in a low ΔQ, and vice versa. However, a different trend is seen for NH3 on 1H-HfS2 and 1H-ZrS2. Interestingly, the value of Eads for NH3 on 1H-ZrS2 is lower than that on 1H-HfS2 and 1H-ZrSe2. However, further charge transfer occurs in the case of NH3 on 1H-ZrS2. Similarly, NH3 on 1T-ZrS2 depicts a high Eads, but more charge transfer occurs for NH3 on 1T-HfS2. In the case of NO2 adsorption, only HfTe2 depicts a contrary behavior. The charge transfers are quite low with respect to Eads, as compared with the remaining Zr (Hf) dichalcogenides.
To gain further insight into the NH3(NO2)–TMDC interaction, we investigated the charge-transfer mechanism between them. Figure 7 and Figure 8 depict charge density difference images for NH3 and NO2 adsorbed on the 1H- (1T-) Zr (Hf) dichalcogenides. The red region shows the charge accumulation, while the green region represents the charge depletion. Different TMDCs polarize differently upon adsorption of NH3 (NO2), and the 1H surfaces show different polarizations than the 1T surfaces. Additionally, electrostatic interactions play a role in the attractive interaction. Researchers clearly observed that a charge accumulation occurs due to the adsorption of NH3 on either of the 1H (Figure 7) or 1T (Figure 8) surfaces, suggesting NH3 acts as a donor. Nevertheless, the NO2 adsorption shows the opposite behavior. On the 1H surfaces, NH3 adsorption shows a moderate charge density difference on 1H-HfS2 and 1H-ZrSe2, giving rise to a moderate interaction energy and a comparatively low charge density difference on the remaining surfaces. This explains why the former exhibits larger adsorption energies (−647 and −518 meV for NH3 on 1H-HfS2 and 1H-ZrSe2) than the latter (−199, −208, −332, and −208 meV for NH3 on 1H-HfSe2, 1H-HfTe2, 1H-ZrS2, and 1H-ZrTe2) (Table 3 and Figure 6). In contrast, the polarization increases when NO2, instead of NH3, adsorbs on the 1H surfaces. Along with moderate, a comparatively strong polarization is observed on 1H-HfTe2 and 1H-ZrTe2. Conversely, instead of 1T-ZrSe2, 1T-ZrS2 shows a moderate polarization upon adsorption of NH3. The remaining surfaces depict different polarizations compared to the corresponding 1H-surfaces; i.e., the NO2 that adsorbed on the 1T surfaces shows less polarization for all cases.
The electronic properties of NH3 (NO2) adsorbed on the 2D TMDCs were examined. Here, the position of the valance and conduction bands, and the change of the Fermi level, along with the pristine 1H (1T) band positions, were considered. The calculated band structure of NH3 (NO2) adsorbed on the 1H and 1T 2D Zr (Hf) dichalcogenides are plotted in Figure 9 and Figure 10, respectively. Researchers found that the valance band maxima and conduction band minima are not at the same points for all cases, implying they are all indirect band gap materials. Researchers observed that the adsorption of NH3 molecules does not introduce any additional states within the band gap of the pristine 1H (1T) Zr (Hf) dichalcogenides. Furthermore, 0.001–0.04 eV changes were seen at the positions of the conduction and valance bands for most materials, which are almost insignificant and consistent with the adsorption of NH3 on other metal chalcogenides [35]. The conduction bands of 1H-HfS2, 1H-ZrS2, and 1T-ZrS2 depict an upshift of 0.048, 0.077, and 0.043 eV, respectively, from the Fermi level. All the NH3-adsorbed material bands features are similar to those of the pristine materials, except for 1H-HfS2, 1T-HTe2, and 1T-ZrTe2, and an observable conduction band degeneracy is seen for 1H-HfS2. Interestingly, gapless 1T-HTe2 and 1T-ZrTe2 depict band openings (~0.1 eV) with clearly separated conduction and valance bands after NH3 adsorption, although the band gap values are very small.
Conversely, no influence of NO2 adsorption on the band structures of 1T-HTe2 and 1T-ZrTe2 is seen. NO2 adsorption induces a state within the band gap of Zr (Hf) dichalcogenides, contrary to that happens after NH3 adsorption. In 1H-HfS2 (1H-ZrS2), NO2 induces a state that crosses the Fermi level. Other state levels appear just above EF. The states and conduction bands are so near (Figure 9) that the electron can easily move to-and-fro. Therefore, the estimated bandgaps of 1H-HfS2 and 1H-ZrS2 are less than those of the pristine ones. In 1H-HfSe2, a single state at −0.1 eV in-between the bandgap is observed with a slight downshifting of the conduction band (0.122 eV), and a comparatively large valance band shift (0.138 eV) results in bandgap enhancement. 1H-ZrSe2 depicts features similar to those seen for 1H-ZrS2; however, no conduction band expansion occurs. Two additional states cut the EF, where one appears below EF in the case of 1H-HfTe2. No significant shifting in the bands occurs. Like 1H-ZrS2 and 1H-ZrSe2, the 1H-ZrTe2 band is just above the Fermi level, with conduction bands near to these levels, while two states cut EF in 1H-HfTe2, and one appears just below EF. These states are near to the valance band. Easy to-and-fro movement of the electrons from these states to the conduction band is observed for NH3 adsorbed on the 1H Zr (Hf) dichalcogenides, except 1H-HfSe2 and 1H-HfTe2, where electron transfer occurs from the valance band. The NO2-adsorbed 1T Zr(Hf) dichalcogenides bands are identical to those of the pristine materials, but with additional band states. A single state between EF and the conduction band appears in the NO2-adsorbed 1T-HfS2, while the band positions remain unchanged. Similar phenomena is observed for 1H-HfSe2, but with an upward shift in the band positions. Furthermore, a single state appears just below the conduction band in the NO2-adsorbed 1T-ZrS2, where a minor upward shift in the valance band results in a decrease in the band gap. The bands of 1T-ZrS2 shift like those of 1H-HfSe2, where the two states cut EF.

4. Conclusions

The molecular adsorption of NH3 and NO2 gas molecules on 1H and 1T 2D Zr (Hf) dichalcogenides (S, Se, Te) was carried out using DFT. The optimized lattice parameters of 1H were slightly smaller in all cases, compared to those of the 1T Zr (Hf) dichalcogenides. The Zr (Hf) telluride possesses semimetal features in the 1T-phase and semiconductor features in the 1H-phase. The band gaps of the Zr (Hf) dichalcogenides were found to be dependent not only of the metal species Zr and Hf but also of chalcogen, which decreases from S to Se to the Te chalcogen. Both NH3 and NO2 adsorbed exothermically on the 1H and 1T 2D Zr (Hf) dichalcogenides. Noticeably, the adsorption energy was consistently different on the two different phases of the Zr (Hf) dichalcogenides. Researchers observed that NH3 donates a charge to the surface, while NO2 accepts charge on adsorption. Moreover, a single NO2 molecule accepts comparatively more charge than a single NH3 molecule donates. Moreover, a direct correlation between the adsorption energy and charge transfer was seen, except for NH3 on 1H-HfS2 and NO2 on HfTe2. The adsorption of NO2 on HfS2, ZrS2, HfSe2, and ZrSe2 exhibited significant effects on the conduction and valance bands, thereby affecting the band gaps. Our results showed that the electronic properties of 2D Zr (Hf) dichalcogenides can be precisely tuned by the molecular adsorption of NH3 and NO2, which may be useful for future electronic devices.

Author Contributions

Formal analysis, investigation, writing—original draft preparation, visualization, S.S.R.; data curation, methodology, validation, writing—review and editing, A.S.A.; conceptualization, software, supervision, project administration, funding acquisition, B.S. All authors have read and agree to the published version of the manuscript.

Funding

This work was supported by the Hongik University new faculty research support fund.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. 2D Zr (Hf) dichalcogenides, (a) trigonal prismatic (1H) and (b) octahedral (1T) sheet structure, where blue color represents metal (Zr or Hf) and yellow (red) color represents chalcogen (S, Se or Te); (c) 1H arrangement (ABA staking), (d) 1T arrangement (ABC staking).
Figure 1. 2D Zr (Hf) dichalcogenides, (a) trigonal prismatic (1H) and (b) octahedral (1T) sheet structure, where blue color represents metal (Zr or Hf) and yellow (red) color represents chalcogen (S, Se or Te); (c) 1H arrangement (ABA staking), (d) 1T arrangement (ABC staking).
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Figure 2. Band structures of pristine 2D Zr (Hf) dichalcogenides, (top) 1H and (bottom) 1T: [(a),(g)] HfS2, [(b),(h)] HfSe2, [(c),(i)] HfTe2, [(d),(j)] ZrS2, [(e),(k)] ZrSe2, and [(f),(l)] ZrTe2.
Figure 2. Band structures of pristine 2D Zr (Hf) dichalcogenides, (top) 1H and (bottom) 1T: [(a),(g)] HfS2, [(b),(h)] HfSe2, [(c),(i)] HfTe2, [(d),(j)] ZrS2, [(e),(k)] ZrSe2, and [(f),(l)] ZrTe2.
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Figure 3. Adsorption configurations and orientations of NH3 (top row) and NO2 (bottom row) on chemically stable site of 1H: [(a),(g)] HfS2 [(b),(h)] HfSe2 [(c),(i)] HfTe2 [(d),(j)] ZrS2 [(e),(k)] ZrSe2 [(f),(l)] ZrTe2 monolayer sheet.
Figure 3. Adsorption configurations and orientations of NH3 (top row) and NO2 (bottom row) on chemically stable site of 1H: [(a),(g)] HfS2 [(b),(h)] HfSe2 [(c),(i)] HfTe2 [(d),(j)] ZrS2 [(e),(k)] ZrSe2 [(f),(l)] ZrTe2 monolayer sheet.
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Figure 4. Adsorption configurations and orientations of NH3 (top row) and NO2 (bottom row) on chemically stable site of 1T: [(a),(g)] HfS2 [(b),(h)] HfSe2 [(c),(i)] HfTe2 [(d),(j)] ZrS2 [(e),(k)] ZrSe2 [(f),(l)] ZrTe2 monolayer sheet.
Figure 4. Adsorption configurations and orientations of NH3 (top row) and NO2 (bottom row) on chemically stable site of 1T: [(a),(g)] HfS2 [(b),(h)] HfSe2 [(c),(i)] HfTe2 [(d),(j)] ZrS2 [(e),(k)] ZrSe2 [(f),(l)] ZrTe2 monolayer sheet.
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Figure 5. Adsorption energy comparison of (a) NH3 (b) NO2 on trigonal prismatic (1H) and octahedral (1T) 2D Zr (Hf) dichalcogenides.
Figure 5. Adsorption energy comparison of (a) NH3 (b) NO2 on trigonal prismatic (1H) and octahedral (1T) 2D Zr (Hf) dichalcogenides.
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Figure 6. Comparisons of charge transfers with respect to adsorption energy for (a) NH3_1H (b) NO2_1H (c) NH3_1T and (d) NO2_1T.
Figure 6. Comparisons of charge transfers with respect to adsorption energy for (a) NH3_1H (b) NO2_1H (c) NH3_1T and (d) NO2_1T.
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Figure 7. Charge density difference plots for NH3 (top row) and NO2 (bottom row) interacting with 1H: [(a),(g)] HfS2; [(b),(h)]HfSe2; [(c),(i)]HfTe2; [(d),(j)] ZrS2; [(e),(k)] ZrSe2; and [(f),(l)] ZrTe2 monolayer sheet. The isosurface is taken as 5 × 10−3 e/A3. The red or green color distribution corresponds to charge accumulation or depletion.
Figure 7. Charge density difference plots for NH3 (top row) and NO2 (bottom row) interacting with 1H: [(a),(g)] HfS2; [(b),(h)]HfSe2; [(c),(i)]HfTe2; [(d),(j)] ZrS2; [(e),(k)] ZrSe2; and [(f),(l)] ZrTe2 monolayer sheet. The isosurface is taken as 5 × 10−3 e/A3. The red or green color distribution corresponds to charge accumulation or depletion.
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Figure 8. Charge density difference plots for NH3 (top row) and NO2 (bottom row) interacting with 1T: [(a),(g)] HfS2; [(b),(h)]HfSe2; [(c),(i)]HfTe2; [(d),(j)] ZrS2; [(e),(k)] ZrSe2; and [(f),(l)] ZrTe2 monolayer sheet. The isosurface is taken as 5 × 10−3 e/A3. The red or green color distribution corresponds to charge accumulation or depletion.
Figure 8. Charge density difference plots for NH3 (top row) and NO2 (bottom row) interacting with 1T: [(a),(g)] HfS2; [(b),(h)]HfSe2; [(c),(i)]HfTe2; [(d),(j)] ZrS2; [(e),(k)] ZrSe2; and [(f),(l)] ZrTe2 monolayer sheet. The isosurface is taken as 5 × 10−3 e/A3. The red or green color distribution corresponds to charge accumulation or depletion.
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Figure 9. Band structures of pristine, NH3 and NO2 adsorbed monolayer 1H: (a) HfS2, (b) HfSe2, (c) HfTe2, (d) ZrS2, (e) ZrSe2, and (f) ZrTe2.
Figure 9. Band structures of pristine, NH3 and NO2 adsorbed monolayer 1H: (a) HfS2, (b) HfSe2, (c) HfTe2, (d) ZrS2, (e) ZrSe2, and (f) ZrTe2.
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Figure 10. Band structures of pristine, NH3 and NO2 adsorbed monolayer 1T (a) HfS2, (b) HfSe2, (c) HfTe2, (d) ZrS2, (e) ZrSe2, and (f) ZrTe2.
Figure 10. Band structures of pristine, NH3 and NO2 adsorbed monolayer 1T (a) HfS2, (b) HfSe2, (c) HfTe2, (d) ZrS2, (e) ZrSe2, and (f) ZrTe2.
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Table
Table 1. The optimized lattice parameters (a) of Trigonal prismatic (1H) and Octahedral (1T) Zr (Hf) dichalcogenides calculated by PBE functional while keeping the distance between neighboring layers 20 Å [38,39,40,41].
Table 1. The optimized lattice parameters (a) of Trigonal prismatic (1H) and Octahedral (1T) Zr (Hf) dichalcogenides calculated by PBE functional while keeping the distance between neighboring layers 20 Å [38,39,40,41].
MaterialTrigonal Prismatic
a (Å)		Deviation
with 1T (%)		Octahedral
a (Å) 		Literature
a (Å) 		Deviation (%)
calc.exp’t.calc.exp’t.
HfS23.513.043.623.663.631.090.27
ZrS23.543.013.653.693.661.080.27
HfSe23.642.413.733.783.751.061.61
ZrSe23.692.123.773.803.770.790
HfTe23.802.063.883.95 1.77
ZrTe23.880.773.913.95 1.01
calc.: calculated values from literature, exp’t: experimental values from literature [33,34,35,36].
Table
Table 2. Calculated indirect band gap values for trigonal prismatic (1H) and octahedral (1T) 2D Zr and Hf dichalcogenides (S, Se and Te).
Table 2. Calculated indirect band gap values for trigonal prismatic (1H) and octahedral (1T) 2D Zr and Hf dichalcogenides (S, Se and Te).
MaterialTrigonal PrismaticOctahedralLiterature
PristineNH3NO2PristineNH3NO2calc.exp’t.
HfS21.151.181.031.291.291.241.29 1.96
ZrS21.131.210.941.131.181.201.02 1.68
HfSe20.530.530.680.510.570.510.68 1.13
ZrSe20.850.860.750.410.470.400.42 1.2
HfTe20.380.340.4300.1000 -
ZrTe20.290.280.2500.1300 -
calc.: calculated values from literature, exp’t: experimental values from literature [34,35,36,38,40].
Table
Table 3. The Preferable adsorption site, orientation of gas molecule, adsorption energy (Eads), adsorption-distance (d) and the charge transfer (ΔQ) for NH3 (NO2) on 1H Zr (Hf) dichalcogenides.
Table 3. The Preferable adsorption site, orientation of gas molecule, adsorption energy (Eads), adsorption-distance (d) and the charge transfer (ΔQ) for NH3 (NO2) on 1H Zr (Hf) dichalcogenides.
GasNH3NO2
Surface SiteOrientationEads
(meV)		d(Å)ΔQ (e)SiteOrientationEads
(meV)		d(Å)ΔQ (e)
HfS2TMU−6472.430.015TxU−4562.02−0.179
ZrS2THD−3323.600.152TxU−6663.24−0.199
HfSe2THU−1993.380.045TxU−3992.20−0.220
ZrSe2TMU−5182.490.081TxU−6092.17−0.309
HfTe2THU−2083.560.033TxD−9652.28−0.252
ZrTe2THU−2083.550.009TxD−9422.28−0.622
Where TM: metal (Hf or Zr), TH: Hexagon, TX: chalcogen (S, Se, Te), U and D refer to the orientation of NH3 (NO2).
Table
Table 4. The Preferable adsorption site, orientation of gas molecule, adsorption energy (Eads), adsorption-distance (d) and the charge transfer (ΔQ) for NH3 (NO2) on 1T Zr (Hf) dichalcogenides.
Table 4. The Preferable adsorption site, orientation of gas molecule, adsorption energy (Eads), adsorption-distance (d) and the charge transfer (ΔQ) for NH3 (NO2) on 1T Zr (Hf) dichalcogenides.
GasNH3NO2
Surface SiteOrientationEads
(meV)		d(Å)ΔQ (e)SiteOrientationEads
(meV)		d(Å)ΔQ (e)
HfS2TMU−4472.420.195TxD−2043.30−0.050
ZrS2TMU−5872.440.117TxD−2142.66−0.132
HfSe2TMU−1912.450.128TMD−2794.82−0.257
ZrSe2TMU−3452.480.033TMU−2694.35−0.140
HfTe2THU−1983.580.053TMD−6675.12−0.185
ZrTe2THD−1663.870.018TxD−6723.21−0.619
Where TM: metal (Hf or Zr), TH: Hexagon, TX: chalcogen (S, Se, Te), U and D refer to the orientation of NH3 (NO2).

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Raya, S.S.; Ansari, A.S.; Shong, B. Molecular Adsorption of NH3 and NO2 on Zr and Hf Dichalcogenides (S, Se, Te) Monolayers: A Density Functional Theory Study. Nanomaterials 2020, 10, 1215. https://doi.org/10.3390/nano10061215

AMA Style

Raya SS, Ansari AS, Shong B. Molecular Adsorption of NH3 and NO2 on Zr and Hf Dichalcogenides (S, Se, Te) Monolayers: A Density Functional Theory Study. Nanomaterials. 2020; 10(6):1215. https://doi.org/10.3390/nano10061215

Chicago/Turabian Style

Raya, Shimeles Shumi, Abu Saad Ansari, and Bonggeun Shong. 2020. "Molecular Adsorption of NH3 and NO2 on Zr and Hf Dichalcogenides (S, Se, Te) Monolayers: A Density Functional Theory Study" Nanomaterials 10, no. 6: 1215. https://doi.org/10.3390/nano10061215

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