New Structural Insights into Inert-Exfoliated 2D Metal Diborides

Hexagonal AlB2-type structure
Fig. 1: Hexagonal AlB2-type structure.
TEM characterization of MgB2 nanosheets
Fig. 2: TEM of MgB2 nanosheets.
Quantitative characterization of MgB2 nanoplatelets
Fig. 3: Characterization of MgB2 nanosheets.
Comparison of MgB2 nanosheet dimensions
Fig. 4: MgB2 nanosheet dimensions.
Characterization of MgB2 thin films
Fig. 5: MgB2 thin films.
Nanomaterial stability of MgB2 thin films
Fig. 6: Nanomaterial stability.
Material comparison and electromechanical characterization of MgB2
Fig. 7: Material comparison and characterization.

October 09, 2024 - To expand the use of semimetallic two-dimensional materials, researchers from institutions including Trinity College Dublin, Ulm University, and the University of Chemistry and Technology in Prague have developed a method for inert liquid exfoliation of metal diborides. This technique enables the production of thin films with enhanced stability and metallic conductivity by minimizing oxidation. Using a Langmuir-type deposition, the team created stable MgB2, CrB2, and ZrB2 films that show potential as flexible electrodes in printed electronics.

Liquid-exfoliated two-dimensional (2D) materials have emerged as critical components in various applications, including printed electronics [1–3], sensing [4, 5], and energy storage [6, 7]. This family of 2D materials is particularly notable for encompassing conducting, semiconducting, and insulating members [3, 8], enabling their integration as foundational building blocks for electronic devices. Past studies have demonstrated the potential of all-2D printed electronics, where different 2D materials serve as electrodes, dielectrics, and channel materials [1, 3]. Despite the abundance of semiconducting 2D materials that can be liquid exfoliated, there remains a limited selection of conducting and insulating options. Conducting materials like graphene [9], MXenes [10], and silver nanoplatelets [12] have been studied extensively. However, solution-processed graphene films, for instance, generally exhibit modest conductivities of 103–105 S/m [13, 14].

The bonding nature of metal diborides distinguishes them from other materials like graphite. Within a metal diboride, covalent bonding dominates between boron atoms, while interlayer bonding with ionized magnesium atoms exhibits metallic properties due to delocalized electrons [15]. In graphite, by contrast, covalent bonding is confined to the in-plane carbon lattice, and out-of-plane π-bands contribute to its more pronounced two-dimensional electronic character [16]. These differences lead to relatively strong interlayer interactions in metal diborides compared to the weaker van der Waals forces in graphite. This anisotropy in graphite facilitates its efficient exfoliation into high-aspect-ratio 2D nanosheets [17]. For metal diborides, however, the anisotropy in binding strength is expected to be less pronounced.

Due to the reduced anisotropy in binding energy, it has been hypothesized that metal diborides would exfoliate into nanosheets with lower length/thickness aspect ratios [17]. While previous studies have explored the sonication-assisted exfoliation of metal diborides in liquid media [18, 19], quantitative data on their dimensions and electronic properties remain sparse. Moreover, these materials are known to be highly sensitive to oxidation and hydroxylation in ambient or aqueous environments, necessitating controlled processing [18–20].

Building on this foundation, the researchers conducted an in-depth study to provide quantitative insights into the size, electronic properties, and reactivity of pristine sonication-assisted liquid-phase exfoliated metal diborides (Fig. 1). They exfoliated MgB2 powder in dry, degassed isopropanol and compared its behavior under ambient and inert conditions. Aberration-corrected transmission electron microscopy (AC-TEM) confirmed the crystallinity of the nanoplatelets produced under inert conditions. Additionally, the team demonstrated the feasibility of Langmuir-type deposition for fabricating thin films with high metallic conductivity. Films and nanosheet inks exposed to ambient conditions showed signs of decomposition over time, reflected in declines in optical extinction and electrical conductivity. To counteract this, the researchers utilized encapsulation techniques to stabilize the films, effectively extending their usability.

Focusing on inert-processed nanosheets, the team evaluated the preservation of crystallinity through TEM and selected-area electron diffraction (SAED). Samples were deposited via drop casting under inert conditions. TEM imaging revealed a variety of particle sizes (Fig. 2), while SAED patterns indicated that thicker particles tended to retain crystallinity, whereas thinner particles exhibited amorphous characteristics. The researchers applied corrections based on correlations between TEM and AFM statistics to ensure reliable measurements, accounting for potential tip broadening effects [21–25].

Statistical analysis of the step height extracted from multiple nanosheets revealed that the measured step height clusters in multiples of 0.8 nm (Fig. 3E). This value corresponds to the apparent thickness of the thinnest MgB2 structure, which is assumed to consist of alternating magnesium and boron layers. As a result, this step height serves as a reliable indicator for calculating the number of monolayers per nanosheet (N). This approach provides a basis for detailed characterization of the material's structural properties.

The researchers employed careful statistical methods to analyze large datasets, enabling a precise determination of nanosheet size and thickness distributions. This approach is essential for developing theoretical frameworks and correlating nanosheet properties with experimental findings, either in dispersions [6, 26] or deposited films [27]. For MgB2, a scatter plot of nanosheet length (L) versus layer number (N) based on data from over 1,000 individual nanosheets showed that thinner nanosheets are typically smaller in size and vice versa, consistent with observations for other sonication-assisted liquid-phase exfoliated materials (Fig. 3F). Length and layer number histograms (Figs. 3G, 3H) revealed log-normal distributions, typical for this type of nanomaterial.

To further evaluate the exfoliation process, the researchers determined the yield of exfoliated MgB2 material by gravimetric weighing, finding it to be 21% of the initial mass. While this yield might appear low, they noted that the unexfoliated material removed during the first centrifugation step could be reused in subsequent exfoliation processes, as demonstrated for other material systems [9, 29, 30]. The team also measured wavelength-dependent optical extinction (ε(λ)), absorbance (α(λ)), and scattering coefficients (σ(λ)) for the exfoliated material (Fig. 3I). The broad, featureless extinction spectra highlighted the metallic nature of MgB2. These optical coefficients not only facilitate standardized ink preparation but also provide insights into the nanomaterial’s size and thickness distributions.

A power-law relationship between the average nanosheet area and the layer number was also observed, consistent with previous reports [6, 17, 28] (Fig. 4). This finding further supports the hypothesis that nanosheet dimensions are intrinsically linked to their thickness, a characteristic feature of exfoliated nanomaterials. Such correlations are invaluable for optimizing exfoliation processes and tailoring material properties for specific applications.

The crystallinity and orientation effects in the deposited nanomaterial were investigated using Langmuir-type deposition on a TEM grid, followed by SAED measurements (Fig. 5A, B). The researchers observed that the nanosheets exhibited edge-to-edge alignment with occasional partial overlaps (Fig. 5B). SAED measurements on individual nanosheets revealed that the crystallinity remained unaffected by the deposition steps (Fig. 5B, inset). The data was further analyzed in the form of an extinction histogram (Fig. 5C), which displayed a Gaussian distribution. Homogeneity of the sample was assessed using the coefficient of variation, calculated as the ratio of the standard deviation to the mean. For the histogram in Fig. 5C, this value was determined to be 0.05, indicating that the standard deviation of the thickness distribution was 5% of the mean thickness. To ensure better contact during electrical measurements, the contact pads were reinforced with silver paint (Fig. 5D, inset). The average I−V characteristics across different electrode spacings were recorded (Fig. 5D), and the network resistance was plotted as a function of channel length, L (Fig. 5E). Individual resistance measurements are indicated as red dots in Fig. 5E. The conductivity of the nanosheet network was notably high for solution-processed materials and competitive with state-of-the-art printed graphene films (~105 S/m).13,14 However, it was somewhat lower than the reported conductivities for MXene films (~106 S/m),34 primarily due to the lower aspect ratio of the diboride nanosheets, and significantly below metallic nanoplatelet networks (~107 S/m),35 which, following sintering and filament formation, are not separated by van der Waals gaps. To further characterize the semimetallic behavior of the MgB2 network, transistor measurements were performed. Unlike semiconducting networks, which show current modulation due to carrier balance, no modulation was observed in the transfer characteristics (Fig. 5F), further confirming the semimetallic electrical properties of these networks.

For the deposited thin films, additional changes in the I−V characteristics were recorded over time. The results from the photospectroscopic measurements revealed systematic changes in the nanosheet dispersion and thin films when exposed to ambient conditions. For the dispersion, a steady change was observed across the spectroscopic response (Fig. 6A). Similarly, thin films fabricated through Langmuir−Schaefer deposition exhibited comparable time-dependent changes (Fig. 6B). Kinetic analysis indicated that the decomposition of the dispersion adhered to a first-order rate law (Fig. 6C), whereas the decomposition of the thin films followed a second-order rate law (Fig. 6D). To further interpret the results, the researchers applied an empirical single exponential function, enabling an estimation of the portion of reacted material (PoR = ε1/(ε0 + ε1)) (Fig. 6E). These observations align with earlier reports on the decomposition kinetics of other 2D nanomaterial systems.6,25,36,37 The I−V characteristics of Langmuir−Schaefer films were analyzed after single and iterative depositions (Fig. 6F, G). Increasing the film thickness through iterative deposition did not enhance the film conductivity (Fig. 6H).

Given the significant sensitivity of metal diboride nanosheets to environmental oxidation, developing protective strategies becomes essential. The researchers addressed this challenge by utilizing a spray-on polymer as a protective encapsulation layer. I−V measurements of MgB2 thin films were performed before and after polymer encapsulation, with subsequent tracking over two weeks of exposure to ambient conditions. The I−V curves, displayed in the inset of Fig. 7D, illustrate a drop in conductivity immediately following the polymer application, which the researchers attribute to morphological changes during the deposition process. Notably, after this initial decline, the conductivity stabilized over a period exceeding 300 hours (Fig. 7D). This outcome highlights the potential of encapsulation methods to preserve the properties of sensitive nanomaterials. As conductive nanomaterials are a critical component for electrode applications in the realm of printed flexible electronics,38,39,40 the stability provided by these methods represents a key advancement for practical applications.

Resource: Synnatschke, K., Müller, A., Gabbett, C., Mohn, M. J., Kelly, A. G., Mosina, K., Wu, B., Caffrey, E., Cassidy, O., Backes, C., Sofer, Z., Kaiser, U., & Coleman, J. N. (2024). Inert Liquid Exfoliation and Langmuir-Type Thin Film Deposition of Semimetallic Metal Diborides. ACS Nano, 18(42), 28596−28608.

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