Highly concentrated dispersion of methyl-terminated germanane by liquid exfoliation

Graphene analogs of group IV elements, which are novel two-dimensional (2D) and single-layer nanomaterials, have opened up new horizons in both fundamental science and advanced technology due to their unique electronic, chemical, and electrochemical properties.^1–3) The recent development of a facile approach to synthesis has resulted in crystals of hydrogen-terminated germanane (GeH),⁴⁾ methyl-terminated germanane (GeCH₃),⁵⁾ and alkyl-terminated germananes.⁶⁾ GeH and GeCH₃ are of particular interest due to their high charge mobility and direct band gap.^7–13) The theory about germananes predicts little change in the electronic structure between an isolated sheet and the two-layer unit cell. Therefore, these properties are predicted to exist in exfoliated germanane, which can result in flexible field-effect transistors and an enhancement in photoluminescence quantum yield. GeCH₃ exhibits a sharp photoluminescence band with high quantum efficiency in the visible region,⁵⁾ but little has been reported on its electroluminescence (EL) devices. Furthermore, alkyl-terminated germananes are organic–inorganic hybrid materials, which provide promising nanostructures for the variation of peak wavelength in photoluminescence by fine-tuning their surface chemistry without affecting their unique optical properties.^14–18)

For practical devices with a large area, a scalable and controlled exfoliation technique is required. According to the increasing demand for preparing large quantities of dispersed germanane cheaply and easily, several methods have been established to exfoliate GeH via sonication in appropriate organic solvents,^19,20) surface chemical modification,²¹⁾ and electrochemical delamination.²²⁾ A concentration of 0.2 wt% for GeH dispersions has been achieved in 1,3-dioxolane.²⁰⁾ We recently found that ethyl-terminated layered germanane (GeC₂H₅) is well dispersed in chlorobenzene solvent by liquid exfoliation and fabricated an EL device with a cast film from the dispersion of GeC₂H₅.²³⁾

In this paper, we report the successful fabrication of stable and sufficiently concentrated dispersions of layered GeCH₃, which results in GeCH₃ concentrations above 1.0 mg ml⁻¹ in halogenated solvents. GeCH₃ is well exfoliated in the dispersion and stands more stably than GeC₂H₅. In addition, the germanium sheet is not oxidized during the liquid exfoliation, which allows a facile device fabrication process simply by casting thin films on substrates.

2.1. Materials

GeCH₃ was synthesized by reacting CaGe₂ with methyl iodine in acetonitrile at room temperature for more than one week, as shown in Fig. 1, following a method from the literature.²⁴⁾

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2.2. Preparation of dispersion

2.3. Film preparation

2.4. Characterization

First, we investigated the effect of solvents on the liquid exfoliation of GeCH₃. Sonication was carried out for 48 h at 120 W. Figure 2 shows photographs of the dispersions in various solvents kept for another 48 h in the ambient environment after sonication. The color of the GeCH₃ dispersions in chlorobenzene became deeper by increasing the sonication time. We found that the concentration of the dispersion for GeCH₃ is optimal for halogenated solvents and other solvents give poorer results.

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The concentration of stably exfoliated GeCH₃ in the dispersions was estimated by taking a 1 ml aliquot of each dispersion after it was kept for 48 h in the ambient atmosphere, and evaporating the solvent in a vacuum-drying oven. The results together with the Hansen solubility parameter of each solvent are summarized in Table I. Hansen solubility parameters are related to the dispersive (δ_D), polar (δ_P), and hydrogen bonding (δ_H) contributions to the cohesive energy density of the material and measure the energy required to disperse one phase in another.^25,26) Interestingly, successful dispersions are achieved for halogenated solvents with relatively large δ_D and small δ_P and δ_H values. The parameter δ_D correlates with the polarizability of atoms and functional groups.²⁷⁾ The polarizability values of germanium and chlorine atoms (5.84 × 10⁻²⁴ and 2.18 × 10⁻²⁴ cm³) are larger than those of carbon, nitrogen, and oxygen atoms (1.67 × 10⁻²⁴, 1.10 × 10⁻²⁴, and 0.802 × 10⁻²⁴ cm³).^25,26) Higher polarizability is one of the important properties of germanium atoms: a halogenated medium with larger δ_D may reduce the agglomeration of germanium sheets and assist in maintaining a stable suspension. Interestingly, the effective solvent for dispersions of GeCH₃ was found to be different from that of GeH reported by Nakamura et al.²⁰⁾ Further analysis is ongoing to clarify the mechanism of dispersion.

Figure 3(a) shows the sonication time dependence of the concentration for the GeCH₃ chlorobenzene dispersion liquid-exfoliated at a sonication power of 180 W. After the resulting dispersions were left to stand for 48 h in the ambient atmosphere, the concentration was estimated by evaporating the dispersion solvent and weighting the residual powder. The results of several cycles were plotted. The GeCH₃ powder already started being exfoliated and dispersed within a short sonication time such as 1–2 h, but rapid sedimentation occurred within 48 h of standing. The further increase of sonication time suddenly afforded a more stable and concentrated dispersion. The mechanism of this interesting "induction time" is under investigation. The stability of the dispersions was further monitored after settling for two months in the ambient environment. As shown in Fig. 3(a), a slight decrease in concentration was observed with time, yet GeCH₃ remained dispersed for over half a year. The GeCH₃ dispersions afforded much less sedimentation than the GeC₂H₅ dispersion, which became a clear solution within a month, perhaps due to the difference in the interlayer between germanium sheets.

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Next, we attempted dispersions of GeCH₃ in chlorobenzene (Cl-Ph) and 1,2-dichlorobenzene (DCl-Ph) for a range of sonication times at 120 W. The concentration after the dispersion was kept for 48 h in the ambient atmosphere was plotted. For both of the solvents, the concentration of dispersed GeCH₃ became higher with increasing sonication time, reaching 0.7 and 1.2 mg ml⁻¹ for Cl-Ph and DCl-Ph, respectively, as shown in Fig. 3(b). In addition, DCl-Ph showed significant advantage on sonication time.

The structures of the cast films of exfoliated GeCH₃ were investigated by FT-IR and Raman spectroscopy, as well as by XRD measurements and scanning electron microscopy (SEM). The chlorobenzene dispersions of GeCH₃ were fabricated with a sonication power of 100 W for 3 days. A small aliquot of the supernatant was picked up and dropped onto Au-evaporated glass plates for IR RA measurements, onto glass substrates for Raman and XRD measurements, and onto a SiO₂/Si wafer for SEM analyses, followed by evaporation of the solvent in air. The data on the GeCH₃ powder are also plotted in Figs. 4(a)–4(c). The FT-IR spectrum of the films cast from the exfoliated GeCH₃ dispersions [Fig. 4(a)] exhibits peaks corresponding to CH₃ stretching around 2900–3000 cm⁻¹, CH₃ bending around 1200–1400 cm⁻¹, and CH₃ rocking around 800 cm⁻¹, while a peak due to Ge–H stretching around 2000 cm⁻¹ is not observed. The Raman spectrum of the exfoliated GeCH₃ cast film exhibits an in-plane Ge–Ge framework vibration (E₂ mode) peak at 304 cm⁻¹, as shown in Fig. 4(b). The peak is blue-shifted compared to that of the bulk GeCH₃ powder, which is probably due to the size effect of the germanium nanosheets exfoliated by liquid exfoliation.²⁸⁾ In addition, the absence of a strong peak around 440 cm⁻¹ indicates that the GeCH₃ sheets were not oxidized during the sonication.²⁹⁾ Figure 4(c) shows the XRD pattern of the film cast on glass, together with that of the bulk powder. The peak labeled with an asterisk of the pattern for the bulk powder shows residual germanium in the sample. The film from the exfoliated GeCH₃ dispersion shows a single, prominent peak corresponding to the (002) reflection with a 9.61 Å interlayer distance between the germanium sheets. The broad peaks around 6° and 27° of the cast films are due to the glass substrate. SEM images reveal that 2D plates with lateral dimensions of a few hundred nanometers are lying in a disordered manner. The size of the 2D plates depends on the sonication power and time, accompanied by smaller sheets around the larger plates in a nearly unseparable manner. AFM images show several overlapping plates with a thickness of a few tens of nanometers, which are probably composed of a few tens of layers of GeCH₃ sheets. These results clearly confirm that sonication leads to successful exfoliation into a few tens of layers of GeCH₃ sheets along the layer plane due to weak van der Waals forces between methyl groups.

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When the GeCH₃ dispersions were irradiated with a handheld 365 nm light, the photoluminescence could be easily detected by eye, as shown in Fig. 5. Figure 5 shows the photoluminescence spectra of the GeCH₃ dispersions and the film cast on glass from the GeCH₃ dispersions. For comparison, the UV–visible absorption spectrum is also plotted in Fig. 5. The GeCH₃ dispersion was diluted with CHCl₃ when used for the UV–visible absorption spectrum measurement to avoid scattering loss. The UV–visible absorption spectrum of the GeCH₃ dispersion has no distinct peaks and its intensity monotonically increases toward higher energy. The photoluminescence spectra of the GeCH₃ dispersion and the cast film start rising from the absorption edge and show a single peak at 1.83 and 1.80 eV, respectively. This small difference in emission wavelengths is perhaps due to the influence of the substrate.³⁰⁾ However, the spectral features of both are similar with a full width at half-maximum of 252 meV, which is in good agreement with those of GeCH₃ flakes exfoliated with Kapton tape.⁵⁾

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We have found that dispersions of methyl-terminated layered germanane (GeCH₃) are stable for more than half a year in halogenated solvents. The dispersion concentration increases with the sonication time, depending on the kind of halogenated solvent. This scalable process of GeCH₃ dispersions by liquid exfoliation makes them attractive for novel electronic and optoelectronic devices such as EL devices, nonvolatile memory, solar cells, and film transistors which remain to be explored.

This work was supported by JSPS KAKENHI Grant Nos. JP18K04253 and JP19K05607.