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

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.

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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.

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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.

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