AVS 68 Session 2D+AS+SS-MoA: 2D Materials: Defects, Dopants, Edges, Functionalization, and Intercalation

Two-dimensional (2D) materials are one of today’s hot topics in the semiconductor industry due to the fact that they can be used to fabricate nanodevices with attractive properties [1, 2]. However, they may lose their features in the presence of defects. For instance, 2D hexagonal boron nitride (hBN) is an excellent insulator but defects lower its breakdown voltage in the experiment [3]. Therefore, the aim of this study is to understand the mechanism of the dielectric breakdown in hBN as compared to amorphous boron nitride (aBN), using first-principles calculations based on density functional theory. The CP2K quantum chemistry and solid-state physics software package is used (quickstep algorithm based on the Gaussian and plane waves method) [4, 5]. The generalized gradient approximation of Perdew-Burke-Ernzerhof [6] is employed, and Grimme's dispersion correction [7, 8] with Becke-Johnson damping [9] is used as the van der Waals forces play a significant role in 2D materials. The obtained densities of states show that the size of the band gap decreases for increasing defect density, confirming that 2D materials are sensitive to defects. We explain the experimental observation that the breakdown voltage decreases from hBN (crystalline) to aBN (amorphous).

-------------------------------------------------------------------------------

[1] Y. Shi, X. Liang, B. Yuan, V. Chen, H. Li, F. Hui, Z. Yu, F. Yuan, E. Pop, H. S. P. Wong, and M. Lanza, Nat. Electron. 1, 458 (2018).

[2] C. Wen, X. Li, T. Zanotti, F. M. Puglisi, Y. Shi, F. Saiz, A. Antidormi, S. Roche, W. Zheng, X. Liang, J. Hu, S. Duhm, J. B. Roldan, T. Wu, V. Chen, E. Pop, B. Garrido, K. Zhu, F. Hui, and M. Lanza, Adv. Mater. 33, 2100185 (2021).

[3] Y. Shen, K. Zhu, Y. Xiao, T. Knobloch, D. Waldhör, A. H. Basher, F. Saiz, S. Pazos, X. Liang, W. Zheng, Y. Yuan, J. B. Roldan, U. Schwingenschlögl, H. Tian, H. Wu, T. Grasser, and M. Lanza, submitted to Nat. Electron. (2022).

[4] J. Van de Vondele and J. Hutter, J. Chem. Phys. 127, 114105 (2007).

[5] J. Hutter, M. Iannuzzi, F. Schiffmann, and J. Vandevondele, Wiley Interdiscip. Rev.: Comput. Mol. Sci. 4, 15–25 (2014).

[6] J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996).

[7] S. Grimme, S. Ehrlich, and L. Goerigk, Theory. J. Comput. Chem. 32, 1456−1465 (2011).

[8] S. Grimme, J. Antony, S. Ehrlich, and H. Krieg, J. Chem. Phys. 132, 154104 (2010).

[9] E. R. Johnson and A. D. Becke, J. Chem. Phys., 123, 024101 (2005).

Two-dimensional semiconductors such as WS₂ are promising materials for use in next-generation nanoelectronics due to their high mobility and scalability allowing for ultra-short gates. Doping is required to make the p-type FETs needed for CMOS logic and doping can also reduce the contact resistance, which is another important challenge to overcome for transistors based on 2D materials.¹ Separately, doped 2D materials are receiving interest as affordable catalysts, as the dopants enhance the catalytic activity of the basal plane in addition to the already high activity of the edge sites.²

Essential for both electronic and catalytic applications is the precise control over the amount and distribution of dopants in the film. The synthesis method of atomic layer deposition (ALD) allows excellent control over these parameters such that e.g. graded doping profiles and edge decorations can be achieved.³

In this work we synthesize Nb-doped WS₂ by plasma-ALD and characterize the electronic and catalytic performance of the films.Conventional supercycles of type (AC)_m(BC)_n were used to achieve excellent composition control from W-rich to Nb-rich films. Optimal hydrogen evolution reaction (HER) activity was found for Nb-rich films (~85% Nb). Beyond composition tuning, we investigate the effect of different edge terminations on the HER activity by modifying the cycle order in the ALD process. For electronic applications where low dopant concentrations are relevant, we use a modified (AC)_m(ABC)_n scheme, where adsorbed W precursor largely inhibits adsorption of the Nb precursor. This allows for better dopant distribution than conventional supercycles, thereby improving the resistivity and Hall mobility of the films by a factor of two. Further characterization of devices based on the grown films is presented.

These results confirm that ALD-grown Nb-doped WS₂ is a promising material for both electronics and catalysis, and that the use of advanced doping schemes can further improve the performance of these films. This also serves as a demonstration of inhibition-assisted​ doping using ALD, which could be of interest for other material systems.

1. Feng, L., et al. Nanoscale 8, 6507–6513 (2016).

2. Tang, L. et al. Adv. Funct. Mater.31, 2006941 (2021).

3. Schulpen, J. J. P. M. et al. 2D Mater.9, 025016 (2022).

Transmission electron microscopy (TEM) is used for understanding the local structure of nanomaterials. Although, we are frequently concerned about understanding behaviour during chemical reactions or while undergoing physical processes, most TEM is performed with the sample exposed to high vacuum, which can change the atomic structure of surfaces and interfaces. Unfortunately, commercial in-situ liquid, gas or electrochemical cell TEM imaging holders often severely limit atomic resolution imaging and chemical analysis.

For investigating the chemical reactivity and degradation of 2D materials without exposing them to the TEM vacuum, an effective approach is to encapsulate the sample between two inert and impermeable few-layer 2D sheets (such as graphene or hexagonal boron nitride). This also enables transfer of air sensitive specimens from an inert vacuum or glove environment to the TEM without exposing the material to atmospheric conditions. We have applied this approach to study air sensitive 2D materials, including local point defects, doping and edge structures in a wide range of structures (e.g. CrBr3, GaSe, black phosphorus) (Fig. 1c)[1,2]. We also find that encapsulation with inert 2D materials is an effective route to preserve the delicate surface structure of hydrated 2D materials, enabling the visualization of exchangeable surface cations on few layer clays and micas [3] and a route to understand the changes in atom/ion motion at interfaces where the 2D materials are twisted with respect to each other (Fig. 1d).[3,4]

This 2D heterostructure approach can also be used to investigate solid-liquid interfaces. Building on nanochannel technology developed by the group of Andre Geim (Fig. 1b)[5] we have developed in-situ liquid phase TEM imaging using 2D heterostructure nanochannels. The in-situ 2D heterostructure liquid cell approach provides atomic resolution imaging and analysis and makes it possible to study the earliest stage of chemical synthesis [6]. It also reveals the large differences in adatom adsorption sites on 2D surfaces in vacuum compared to hydrated environments and allows study of dynamic adatom motion at solid liquid interfaces [7]; something that was not previously possible by any technique (Fig. 1a).

[1] Hopkinson, et al. ACS Nano, (2019), 13, 5, 5112; [2] Hamer et al. Nano Lett. (2020), 20, 9, 6582 [3]Zou et al, Nature Materials (2021), 20, (12) 1677; [4] Weston et al, Nature Nanotechnology (2020), 15 (7), 592; [5] A Keerthi et al, Nature, (2018), 558 (7710), 420; [6] Kelly et al, Advanced Materials, (2021) 33, 29, 2100668; [7]Clark et al. https://arxiv.org/abs/2203.04906 (2022);

Mixed valence manganese oxides (MnO_x) have attracted significant research interest in recent years due to the reversible low energy redox reactions between manganese oxidation states (Mn⁺², Mn⁺³, and Mn⁺⁴)¹, which has enabled its use in catalysis², energy storage³, and gas sensing⁴. Of these manganese oxide compounds, manganese dioxide (MnO₂) has been of particular interest due to its wide variety of synthesized structural polymorphs (α (1x2 tunnel), β (1x1 tunnel), γ (spinel), and δ (layered))⁵ which allow for significant control over the active surface area and reactive properties MnO₂. In particular, the Mn⁺³ defect density, which has been found to increase the reactivity of the MnO₂ film⁶, is highly variable between polymorphs, with the δ phase exhibiting the highest defect density. Water stabilized δ-MnO₂ (H-δ-MnO₂) in particular contains the highest number of Mn⁺³ defects due to the presence of Mn^+2/+3 in the interlayer, which neutralize the layer charge from lattice defects⁷. However, the production of H-δ-MnO₂ has proven challenging in the literature. In this work, we present the synthesis of H-δ-MnO₂ on epitaxial graphene silicon carbide via electrodeposition. The electrodeposition was carried out in a 3 electrode electrochemical cell in a 3-step process with a duration of 1.25 seconds. The resulting films were then characterized using a combination of Raman spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM) to confirm the formation of H-δ-MnO₂ and probe its surface morphology. The Raman spectra indicated the successful formation of H-δ-MnO₂, while the AFM and SEM indicated the surface has a nanofibrous character, increases the active surface area of the thin film. We then demonstrated that the material can be converted to Alkali type (Li, Na, K) δ-MnO₂ via intercalation without damaging the EG substrate. After the process, the material was characterized again with Raman, AFM, and SEM, which confirmed the transformation from H-type to Alkali-Type. As the interlayer spacing of δ-MnO₂ and its applications are related to the intercalated ions, this demonstrates the tunability of this heterostructure and its potential to be a platform for a variety of applications, including energy storage and gas sensing.

References: [1]L. Spinelli et al. Sensors 17(7) (2017)[2] F. Cheng et al. Chem. Mater 22(3) (2010)[3] Y.J. Huang et al. Electrochim Acta 99 (2013)[4]N. Joshi et al. Microchim. Acta 185 (2018)[5]Z. Chang et al. Proc Natl Acad Sci USA 115(23) (2018) [6] Julien et al. Spectrochim Acta A 60 (2004) [7] Drits et al. Am Mineralogist 82 (1997)