3.2.1  Growth of calcium-intercalated silicene, CaSi2

CaSi₂, one of the Zintl phases (Zintl 1939; Schäfer et al. 1973), is an intermetallic compound formed between a strongly electropositive metal (e.g., alkali metals and alkaline earth metals) and a somewhat less electropositive atom such as Si. CaSi₂ has a 2D silicon subnetwork resembling buckled Si(111) planes in which the Si₆ rings are interconnected with sp ³ bonds while the puckered (Si⁻) _n polyanion layers are separated by planar monolayers of Ca²⁺. CaSi₂ has a trigonal structure (space group R-3m) at ambient pressure, which can only vary in its stacking sequence to give rise to tr6 (a = 0.3855 nm, c = 3.062 nm) (Evers 1979) and tr3 (a = 0.3829 nm, c = 1.590 nm) (Dick and Ohlinger 1998) polymorphs. In tr6 CaSi₂, the stacking of the trigonal Ca layers follows an AABBCC sequence with a six-layer repeat distance (Figure 3.1a and b). Tr3 CaSi₂ has a three-layer repeat distance, with the Ca layers stacked following an ABC sequence (Figure 3.1c). CaSi₂ ingots typically crystallize in the tr6 polymorph. Thus, it has been reported that only 3 out of more than 200 samples crystallized in the tr3 polymorph (Dick and Ohlinger 1998). Tr6 CaSi₂ is more stable than its tr3 counterpart. Additionally, the calculated Gibbs energy relation suggests that tr6 CaSi₂ stabilizes over tr3 with higher temperature (Nedumkandathil et al. 2015). It has been speculated that impurities and/or defects stabilize the tr3 CaSi₂ phase (Evers 1979; Vogg et al. 1999; Nedumkandathil et al. 2015).

Figure 3.1   (a) HAADF-STEM image of tr6 CaSi2. (b) Model of tr6 CaSi2. (c) Model of tr3 CaSi2.

Several CaSi₂ single-crystal growth experiments have been reported. Single-crystal specimens with lateral sizes of 8 × 8 mm and 5 × 5 mm were obtained by slow cooling of ingots (Evers and Weiss 1974; Yamanaka et al. 1981). Using the floating zone method, a CaSi₂ single crystal with a diameter of 10 mm and length of 100 mm was also fabricated (Hirano 1991). The generally accepted Ca–Si binary phase diagram indicates that the process CaSi + liquid → CaSi₂ occurs as a metastable peritectic reaction in the high-purity melt, on the other hand, the stable peritectic reaction Ca₁₄Si₁₉ + liquid → CaSi₂ takes place at CaSi₂ stoichiometric compositions when the melt contains some impurities (Figure 3.2a) (Yaokawa et al. 2014). A CaSi₂ single phase is formed from the high-purity stoichiometric CaSi₂ melt by using 5N Ca and 5N Si as starting materials while slightly undercooling (by only 5°C). Thus, a tr6 CaSi₂ single-phase ingot can be obtained by casting at 1–100°C/min cooling rates (Figure 3.2b through d).

Figure 3.2   (a) Phase diagram of the Ca–Si binary system around CaSi2 obtained on the basis of differential thermal analysis (DTA) results. Top-surface pictures of ingots fabricated from Ca1.00Si2 melt at varying cooling rates: (b) 1, (c) 10, and (d) 100°C/min, respectively.

3.2.2  Electronic properties of monolayer silicene in CaSi2

CaSi₂, considered a calcium-intercalated silicene, provides a good opportunity to investigate the intrinsic electronic structure of genuine silicene. While the Si atoms in CaSi₂ form a hexagonal 2D layer, silicene synthesized on a silver substrate has been reported to have a different structure (i.e., dumbbell structure) (Cahangirov et al. 2014). In CaSi₂, the Ca atoms are considered to play a role in stabilizing the buckled honeycomb network structure of Si atoms by donating electrons to the silicene layers (Cahangirov et al. 2009; Rui et al. 2013).

The electronic properties (i.e., band structure) of silicene in a tr3 CaSi₂ single crystal can be analyzed by high-resolution angle-resolved photoemission spectroscopy (ARPES). From the X-ray diffraction (XRD) patterns of the powder and the single-crystal samples (Figure 3.3a and b), the grown tr3 CaSi₂ crystal reveals the high-quality crystal. Low-energy electron diffraction (LEED) measurements of the cleaved surface also show a clear 1 × 1 pattern, thereby indicating that no reconstruction of the crystal structure took place at the surface (Figure 3.3c). In the ARPES-derived band structure, a massless Dirac cone of dispersed π electrons at the K(H) point in the Brillouin zone is clearly observed, together with σ-band dispersions at the Γ point (Noguchi et al. 2015). Furthermore, the Dirac point is located at ca. 2 eV from the Fermi level (E _F), thereby revealing a substantial charge transfer from the Ca atoms to the silicene layers (Figure 3.3d through f). The ARPES results indicate that the sp ² bonding framework essentially holds the CaSi₂ structure, thereby producing the massless Dirac-cone state at the K point despite the strongly buckled structure of the silicene layers (i.e., the graphene-like electronic structure is stably formed in this metal-intercalated multilayer silicene).

Figure 3.3   (a) XRD pattern from the tr3 CaSi2 crystal powder sample. (b) XRD pattern from the (111) cleavage plane of a CaSi2 single crystal. (c) LEED pattern from the (111) cleavage plane of a CaSi2 crystal measured with a primary electron energy of 80 eV. (d) High-resolution ARPES spectra near the Dirac point at ca. the K(H) point in tr3 CaSi2. (e) ARPES intensity plot around the K(H) point as a function of the wave vector and the binding energy. The peak position in the ARPES spectra, obtained by fitting with Lorentzians, is shown by blue and red circles. (f) Second-derivative ARPES intensity plot around E D.

3.3.1  Synthetic Methods

Experimentally, three types of bilayer silicene can be synthesized from CaSi₂. After annealing in an ionic liquid (i.e., [BMIM][BF₄]) at 250–300°C, a CaSi₂ single crystal is transformed into a CaSi₂F _x (0 ≤ x ≤ 2.3) compound through the diffusion of F⁻. The concentration gradually decreases from the crystal edge to the interior (Figure 3.4a and b). STEM-EDX elemental mapping identifies the dark and bright crystal domains as CaF₂ and Si phases, respectively (Figure 3.4c through g). These planar domains are identified as trilayer CaF₂, trilayer Si, bilayer CaF₂, and a novel bilayer silicene (denoted as w-BLSi in Figure 3.4g and h) in the CaSi₂F_1.8 and CaSi₂F_2.0 compounds shown in Figure 3.4b. Furthermore, two types of bilayer silicenes (i.e., inversion symmetry [i-BLSi] and mirror symmetry [m-BLSi] silicenes) are recognized in the CaSi₂F_0.6–1.0 composition area (Figure 3.4i). The formation of m-BLSi is in accordance with previous MD predictions (Morishita et al. 2008). The i- and m-BLSi structures must be adjacent to a pair of CaF₂ and CaSi₂ crystal layers. The abundance ratio of i-BLSi to m-BLSi is 124:3, as observed in the HAADF-STEM images. Since the calculated energy of i-BLSi is 0.03 eV/atom lower than that of m-BLSi under vacuum conditions, the calculated abundance ratio is qualitatively reasonable.

Figure 3.4   (a) Cross-sectional back-scatter detector (BSE) image of the crystal grain including a CaSi2F x compound. (b) Electron probe micro-analysis (EPMA) quantitative line analysis result along the red arrow in (a). (c–f) STEM-EDX elemental mapping results of the CaSi2F2 composition region. One-element mapping (c: Si, d: Ca, and e: F). (f) Overlapped mapping of Si, Ca, and F. (g) HAADF-STEM image of the STEM-EDX elemental mapping area. (h) An enlarged HAADF-STEM image taken from a CaSi2F2 region in (b); red arrows indicate an F-vacancy site. (i) HAADF-STEM image taken from a CaSi2F0.6–1.0 region in (b); a bright spot contrast, corresponding to the projected atomic positions of m- and i-BLSi, can be observed in the image.

3.3.2  Structural Determination of Bilayer Silicene

Bilayer silicene generally exists as a multiphase compound in CaSi₂F _x . Thus, the lattice constants and atomic positions of w-BLSi cannot be directly characterized by XRD. HAADF-STEM images at high magnification with atomic resolution often suffer distortion owing to specimen drift during scanning. Thus, the atomic structure of the bilayer silicene can be determined from HAADF-STEM images taken at different incident electron beam directions (Figure 3.5a through c). As shown in Figure 3.5d, the w-BLSi structure has 2D translational symmetry and a wavy morphology. This structure consists of two silicenes in alternating chair and boat conformations, which are vertically connected via four-, five-, and six-membered rings. Figure 3.5e through h indicates the corresponding structural models projected in each direction. Since w-BLSi exclusively consists of Si atoms with tetrahedral coordination, the top atom of the five-membered silicon ring possess unsaturated silicon bonds (i.e., dangling bonds). Therefore, compared with monolayer silicene and i- (or m-) BLSi, the density of unsaturated silicon bonds in w-BLSi decreases by 25 and 50%, respectively.

Figure 3.5   (a–c) HAADF-STEM and simulation (insets) images of w-BLSi. (a) [01]w-BLSi and [11] w-BLSi incident directions ([1-10]CaF2). (b) [10]w-BLSi, [11-2]Si, and CaF2 directions. (c) [13]w-BLSi, [11-2]Si, and CaF2 directions. (d) Schematic illustration of the w-BLSi atomic structure. (e–h) Schematic structures projected in each direction in e [01], f [13], g [11], and h [10] directions.

The 2D translational periods of w-BLSi are calculated to be a = 0.661(2) nm and b = 0.382(3) nm, with the two translational axes being normal to each other. The a period of w-BLSi is similar to the triple lattice spacing of d_11-2 in CaF₂ (0.223 nm), while the b period is similar to that of d-110 in CaF₂ (0.386 nm). Thus, the difference between w-BLSi and CaF₂(111) is lower than the observation error. Since the atomic arrangement of the (111) plane in a CaF₂ crystal exhibited threefold symmetry, three equivalent relative rotation angles are observed between w-BLSi and the CaF₂(111) plane (Figure 3.6a). In addition, the angle between the [01]w-BLSi and the [11]w-BLSi directions is nearly 60° (Figure 3.6b). Nearly all the HAADF-STEM images reveals w-BLSi facing the (111) plane of CaF₂, with the F vacancies (red arrows in Figure 3.4h) on the CaF₂(111) surface being recognized at special positions associated with the wavy structure of w-BLSi.

Figure 3.6   (a) Atomic positions in the interface between the w-BLSi (001) and the CaF2 (111) plane. (b) Three equivalent relative rotation angles between w-BLSi and the CaF2(111) plane.

3.3.3  Band structure

The w-BLSi structure apparently resembles that of re-BLSi (Morishita et al. 2011), although their atomic arrangements are clearly different. Ab initio MD calculations were performed for BLSi under the conditions corresponding to the experimentally observed structure (i.e., BLSi sandwiched between two CaF₂ layers with an F-site surface vacancy of 0.5 at the interface). If the MD calculations were started with i-BLSi, this structure would have been immediately transformed to another BLSi structure. The system is subsequently equilibrated, and the resultant BLSi structure is found to perfectly agree with the experimentally observed w-BLSi structure shown in Figure 3.7a. The electronic density of states (EDOSs) for w-BLSi was calculated by the structure in Figure 3.7a, and the decomposed DOS for Si, Ca, and F are shown in Figure 3.7b. The Ca and F bands are located far below the Fermi level, with the valence bands exclusively consisting of Si bands. Unlike a zero-gap semiconductor such as a monolayer (Huang et al. 2013), the band gap of these materials opens to ca. 0.65 eV. The presence of the F vacancies allows the electrons on Ca to be transferred to Si, thereby enhancing the stability of the w-BLSi structure by saturating the dangling bonds. The CaF_{2− x} domains (i.e., ionic crystalline domains) surrounding the Si layers are key to the formation of the w-BLSi structure.

Figure 3.7   (a) Structure of w-BLSi sandwiched between two CaF2 crystals, with vacancies at half of the F sites at the interface; this structure was used to calculate the DOS and was obtained from the transformation of i-BLSi in the ab initio MD simulation and the subsequent quenching process. (b) Decomposed DOS for Si, Ca, and F in the w-BLSi displayed in (a). (c) Plot of multiplication of the Kubelka–Munk function and energy as a function of energy for CaSi2F1.8–2.3 consisting of w-BLSi, trilayer silicene with dangling bonds, and F-terminated trilayer silicenes.

The optical band gap of w-BLSi can be measured from the absorption spectrum of the powder CaSi₂F_1.8–2.3 sample, although this sample is a mixture of w-BLSi, two types of trilayer silicene, and a CaF₂ layer. The DOS for the 2D crystal is constant with energy (D(E) = const.) (in a three-dimensional (3D) crystal: D(E) ≅ E ^1/2). Therefore, the relationship between the absorption coefficient and the band gap energy can be described by the equations αhν = const. (direct gap) and αhν = A (hν − Eg) (indirect gap), where α, h, ν, A, and Eg are the absorption coefficient, Planck’s constant, the light frequency, the proportional constant, and the band gap, respectively (Lee et al. 1969; Gaiser et al. 2004; Mak et al. 2010; Bianco et al. 2013). In the case of the 3D crystal, this equation becomes (αhν) ⁿ = A (hν − Eg), where n = 1/2 and 2 indicate direct and indirect transitions, respectively. Thus, the absorption coefficient of an indirect gap is proportional to the energy, whereas that of a direct gap is constant. The diffuse reflectance spectrum is converted to the Kubelka–Munk function (K-M), which is proportional to the absorption coefficient, as shown in Figure 3.7c. Assuming indirect transitions, the absorption edges of the CaSi₂F_1.8–2.3 compound are 1.08 and 1.78 eV. From previous DFT results on monolayer and multilayer silicenes terminated with atoms (Morishita et al. 2010; Gao et al. 2012), it is conjectured that the band gap of F-terminated trilayer silicenes should be ca. 1 eV within the framework of DFT and the Perdew, Burke, and Ernzerhof technique. It should be noted that DFT calculations using a standard generalized gradient approximation functional tend to underestimate the band gap (by ca. 2/3 in crystal Si). This indicates that the band gap experimentally measured for the trilayer silicene should be ca. 1.5 eV. The band gap for w-BLSi, which was estimated to be ca. 0.65 eV via DFT–PBE calculations, is expected to reach ca. 1 eV in experimental measurements. Therefore, the measured gaps are estimated to be 1.08 and 1.78 eV for w-BLSi and F-terminated trilayer silicenes, respectively.

3.4.1  Exfoliation of CaSi2

The formula of CaSi₂ including the formal charges is Ca²⁺(Si⁻)₂. Thus, the electrostatic interactions between the Ca²⁺ and Si⁻ layers are strong. However, it is very important to decrease the amount of negative charge on the silicon layers to facilitate exfoliation. With this aim, Mg-doped CaSi₂ is used as a starting material to decrease the interaction strength between the Ca and Si layers. Thus, the immersion of bulk CaSi_1.85Mg_0.15 in a solution of propylamine hydrochloride (PA∙HCl) leads to deintercalation of the calcium ions, which is accompanied by the evolution of hydrogen (Figure 3.8a). CaSi_1.85Mg_0.15 is subsequently converted into a mixture of Mg-doped SiNS and an insoluble black metallic solid. A light brown suspension containing SiNS can be separated after the sediment is removed from the bottom (Figure 3.8b). Using X-ray photoelectron spectroscopy (XPS), the composition of the as-obtained SiNS is determined to be Si:Mg:O in a ratio of 7.0:1.3:7.5. The Si:Mg ratio is appreciably lower in the starting material CaSi_1.85Mg_0.15, thereby indicating that exfoliation into individual SiNS occurs preferentially in the section of the silicon layer where magnesium atoms are present.

Figure 3.8   (a) Starting materials for CaSi2. (b) Picture of the SiNS suspension. (c) AFM image of Mg-doped SiNS. (d) Line profile taken along the white line in (c). (e) Schematic illustration of Mg-doped SiNS. (f) Atomically resolved AFM image of SiNS.

The dimensions of the SiNS can be directly determined by atomic force microscopy (AFM), and the sheets are found to be 0.37 nm thick, with lateral dimensions ranging from 200 to 500 nm (Figure 3.8c and d). The crystallographic thickness of SiNS (0.16 nm) is calculated from its atomic architecture, and the difference between this value and that obtained by AFM indicates that the surface of the SiNS is stabilized via capping with oxygen atoms as shown in Figure 3.8e. As revealed by the high-resolution AFM images, the closest distance between atoms (i.e., dot-like marks in the AFM image) is 0.41 ± 0.02 nm (Figure 3.8f), which is slightly larger than the distance between Si atoms in the Si(111) plane of bulk crystalline silicon (0.38 nm). These sheets are considered to be very similar to silicene. The overall exfoliation reaction comprised the following steps: (1) The oxidation of CaSi_1.85Mg_0.15 is initiated by the oxidation of the Ca atoms with PA∙HCl, which is accompanied by the release of PA; (2) The resulting Mg-doped Si₆H₆ is likely very reactive and thus easily oxidized with water to form gaseous hydrogen; (3) Mg-doped layered silicon with capping oxygen atoms is exfoliated by reaction with the aqueous PA solution, thereby resulting in a stable colloidal suspension of SiNS.

The absorption spectrum of the silicon sheets shows a peak at 268 nm (4.8 eV), corresponding to the L→L critical point in the silicon band structure, which is strongly blue shifted with respect to the bulk indirect band gap of 1.1 eV (Figure 3.9). The absorbance at 268 nm is observed to be linearly dependent on the silicon content and excludes a possible association of nanosheets in this concentration range (Figure 3.9, inset). The photoluminescence (PL) spectrum obtained using an excitation wavelength of 350 nm shows an emission peak at 434 nm (2.9 eV). These results directly indicate that the 2D silicon backbone is maintained, since the PL spectra of the silicon quantum dots with diameters lower than 2 nm show a peak at 3 eV that red shift by as much as 1 eV upon exposure to oxygen. Moreover, the absence of PL from defect or trap-state recombination, which typically occurs near 600 nm, supports the hypothesis that the observed PL is produced by direct electron–hole recombination in the silicon sheets, as occurs in silicon nanocrystals. As shown above, the exfoliation of Mg-doped CaSi₂ with PA∙HCl results in SiNS with capping oxygen atoms.

Figure 3.9   Room-temperature optical properties of Mg-doped SiNS. UV/Vis spectra of SiNS suspensions at various concentrations. Inset: the absorbance at 268 nm was plotted against the concentration of SiNS.

3.4.2  Layered silicon compounds

Deintercalation of calcium from the interlayer of CaSi₂ can be achieved using HCl solutions at low temperatures (Figure 3.10). Thus, CaSi₂ changes into amorphous silica with the evolution of significant amounts of hydrogen upon reaction with a 1N HCl solution at room temperature. Treatment with ice-cold concentrated aqueous HCl resulted in CaSi₂ being topotactically transformed into a green-yellow solid (i.e., layered siloxene [Si₆H₃(OH)₃]) with the release of hydrogen gas, as described in Reaction (3.1):

Figure 3.10   Schematic of the reaction of CaSi2 with HCl aqueous solutions.

The crystalline sheets contain puckered 2D Si layers similar to crystalline Si(111) layers, with Si atoms being stabilized by terminal hydrogen or hydroxide groups located out of the layer plane. At temperatures below −30°C, CaSi₂ is transformed into layered polysilane (Si₆H₆) without the release of hydrogen, as shown in Reaction (3.2). This means that additional reactions between the Si layer and water do not occur.

When compared with CaSi₂, the interlayer bonding between adjacent 2D Si layers is weaker in both Si₆H₃(OH)₃ and Si₆H₆ since the bonding takes place by weak hydrogen bonds and van der Waals forces, respectively. Using a solution of HCl in methanol, ethanol, butanol, C₁₂H₂₅, benzyl alcohol, or CH₂COOMe, the corresponding alkoxide-terminated organosiloxenes (Si₆H₅OR, where R = methanol, ethanol, butanol, C₁₂H₂₅, benzyl alcohol, or CH₂COOMe) can also be obtained. However, the organosiloxenes are insoluble in any organic solvent and hence do not exfoliate.

3.5.1  Synthesis and characterizations of Ph-SiNS

The Grignard reaction is one of the important organometallic chemical reactions for C–C and C–Si bond formation (Ashish et al. 1996). In this reaction, aryl-magnesium halides (i.e., Grignard reagents) serve as nucleophiles and react with the electrophilic atoms. Thus, phenyl-modified SiNS (Ph-SiNS) was successfully synthesized by the reaction of Si₆H₆ with phenyl magnesium bromide (Ph-MgBr) in tetrahydrofuran (Sugiyama et al. 2010). Halogenation of the silicon substrates is generally required for Si–C bond formation using Grignard reagents owing to the low reactivity of the Si-H group (Dahn et al. 1993). However, in the case of Si₆H₆, the Si-H groups react with Grignard reagents without halogenation. The as-obtained Ph-SiNS is a colorless paste that dissolves in typical organic solvents. FTIR and ¹H-NMR analyses indicate that the composition of Ph-SiNS is Si₆H₄Ph₂, thereby revealing the substitution of approximately 30% of the Si-H groups with phenyl groups. The XANES spectrum of this material exhibits two peaks at 1841 and 1844 eV, attributed to the Si–Si and Si–C bonds, respectively, with no peak derived from Si–O bonds being observed.

The AFM images of Si₆H₄Ph₂ show a flexible and flat-plane monolayer sheet with 1.11 nm thickness, which is in good agreement with the thickness value of the structural model of Ph-SiNS (Figure 3.12a through c). In addition, atomically resolved AFM images show a periodic arrangement of phenyl groups on the sheet surface as atom-like dots on the Si₆H₄Ph₂ surface (Figure 3.12d). The closest distance between the dots is 0.96 nm, which is in good agreement with the distance between phenyl groups in the Si₆H₄Ph₂ structural model (Figure 3.12e and f).

Figure 3.12   (a) AFM image of Ph-SiNS. (b) Line profile along the black line in (a). (c) Side view of the model structure of Ph-SiNS. (d) Atomically resolved AFM image of the surface of Ph-SiNS. (e) Line profile along the black line in (d). (f) Top view of the model structure of Ph-SiNS.

3.5.2  Optical properties of amino-modified SiNS

An aryl-methyl moiety, having photoelectric characteristics, can also be attached to the surface of Si₆H₆. Amino-modified SiNS surfaces treated with benzene- (Ph-), naphthalene- (Np-), and carbazole- (Cz-) containing amino substituents are prepared by dehydrogenative coupling of Si₆H₆ with the corresponding amines (Ohshita et al. 2016). XRD analysis of the obtained samples reveals a clear diffraction peak at 1.32 nm for Np-SiNS, which is likely ascribed to the stacked structure. In contrast, no clear XRD peaks are observed for Ph-SiNS and NS-SiNS. This is indicative of the strong tendency of Np-SiNS to form stacking structures, likely originating from the extended conjugation of naphthalene.

The obtained samples exhibit different optical characteristics depending on the nature of the aromatic unit. Thus, UV/V is absorption and PL bands produced by both the aromatic units and the SiNS consisting of 2D silicon frameworks are observed in each sample. The strong stacking nature of Np-SiNS is also confirmed from the PL spectrum, which shows a broad band likely ascribed to π stacking. Furthermore, photoinduced currents are observed for the Np-SiNS and Cz-SiNS films, which are prepared on the electrode by a drop-drying method (Figure 3.13). The photocurrent is generated by UV light irradiation, thereby suggesting that the aromatic substituents and/or silicon frameworks absorb the light for photocurrent generation.

Figure 3.13   Photoinduced current generation behaviors of (a) naphthalene- (Np-) and (b) carbazole- (Cz-) modified SiNS films.

3.5.3  Self-stacking properties of amino-modified SiNS

The dehydrogenative coupling reaction can be widely applied to other organoamines for synthesizing other alkyl- or aryl-modified SiNS. Thus, other primary n-alkylamines, such as n-butyl- (C4-), n-hexyl- (C6-), n-decyl- (C10-), n-dodecyl- (C12-), and n-hexadecyl- (C16-) amines, also react with Si₆H₆ to produce the corresponding alkylamine-modified SiNS (Okamoto et al. 2015). The XRD peak attributed to the stacking structure, namely (001) planes, shifts to lower angles depending on the length of the alkyl chain and the interlayer distance [calculated from the (001) reflection] and expands to 1.3–3.35 nm (Figure 3.14a). This result indicates that the interlayer distance can be controlled by the length of the attached alkyl chains. The linear proportional relation between the interlayer distance (d spacing) of the obtained samples and the carbon number of the n-alkylamine suggests that the attached n-alkylamines are regularly arrayed, taking the same conformation. The slope of 0.172 nm suggests a bilayered alkyl chain structure at a tilt angle of ca. 47° with respect to the stacking layers (Figure 3.14b and c).

Figure 3.14   (a) XRD patterns of n-alkylamine-modified SiNS having different lengths of alkyl chain. (b) Relationship between the d space and m for Cm-SiNS stacked structures. Peaks marked with circles are derived from the (001) plane of Cm-HCl salts. (c) Structure model of regularly stacked C6-SiNS.

The reaction of Si₆H₆ with α, ω-alkyldiamines, except for diaminopropane, readily progressed to produce alkyldiamine-modified SiNS (Okamoto et al. 2015). Even in this case, the interlayer distance expands depending on the length of the alkyl chain, but the degree of expansion of the interlayer distance is lower than that of n-alkylamine-derived SiNS. Since both ends of amino groups in the alkyldiamines react with the SiNS, a single-layer structure of alkyldiamine is formed in the interlayer space. The relationship between the interlayer distance and the carbon number in the alkyldiamine suggests a single-layered alkyldiamine structure at a tilt angle of ca. 66° with respect to the stacking layers (Figure 3.15a and b). In addition, the alkyldiamine-modified SiNS shows a low dispersibility against organic solvents, since the SiNS layers should be covalently bonded by the alkyldiamines.

Figure 3.15   (a) Relationship between the d space and m for DiCm-SiNS stacked structures. (b) Structure model of DiC12-SiNS. (c) Approximate structure model of C12COOH-SiNS.

By using alkylamine derivatives such as ω-aminocarboxylic acids, novel functional groups can be introduced on the SiNS surface via the alkyl chains. Thus, 12-aminododecanoic acid (C₁₂COOH) reacted with Si₆H₆ in pyridine to form a carboxylic group-containing alkyl chain fixed on the SiNS surface by Si–N linkages (Okamoto et al. 2015). The XRD pattern indicates that the interlayer distance of 12-aminododecanoic acid-modified SiNS (C₁₂COOH-SiNSs) is approximately half that of n-dodecylamine-modified SiNS. This result implies that C₁₂COOH moieties attached to adjacent silicon layers mutually overlap each other (Figure 3.15c). The introduction of carboxylic groups on SiNS via alkylamine-linkage creates new possibilities for the development of applications such as combinations with other types of layered materials.

3.5.4  Theoretical properties

Theoretical studies of SiNS, such as DFT or tight-binding calculations, can be used to explore possible stable 2D structures of Si, which are often modeled by considering graphene analogs of Si. DFT and ab initio MD simulations were used to investigate the properties of single-layer organo-modified SiNS [Si₆H₄Ph₂]. The calculations can be performed within the framework of DFT using the projector-augmented wave method and the generalized gradient approximation using the exchange–correlation functional PBE as implemented in the Vienna ab initio simulation package.

The optimized structure of the organo-modified SiNS obtained from DFT calculations is presented in Figure 3.16a. The structure is stable, and the model structure for the Si₆H₄Ph₂ nanosheet is highly plausible, because each of the phenyl groups can easily rotate and tilt at 300 K. The band structure (Figure 3.16b) shows that the organo-modified nanosheet has a direct band gap of 1.92 eV, which represents a widened band gap compared with that of bulk Si (0.7 eV) calculated using the same computational parameters. It is interesting to note that the hydrogenated Si single-layer nanosheet shows a comparable indirect band gap of ca. 2 eV, while the Si₆H₄Ph₂ nanosheet shows a direct band gap of ca. 2 eV, thereby demonstrating the significant effect of molecular functionalization (or doubling the thickness of the hydrogenated nanosheet) on its electronic structure.

Figure 3.16   (a) Structure of the relaxed Si6H4Ph2 showing both side views. (Si, C, and H atoms shown in yellow, blue, and white, respectively) (b) Band structure of Si6H4Ph2. The band energy was measured from the highest occupied level at T = 0 K. (c) Total EDOS of the organosilicon nanosheet. (d–f) Orbital-decomposed density of states of Si, C, and H atoms, respectively. The zero of energy was aligned to the highest occupied level.

The total EDOSs is presented in Figure 3.16c through f along with the partial orbital-decomposed EDOS for Si, C, and H atoms. The major contribution to the bands at and just below the highest occupied level is from the Si 2p_x and 2p_y states (Figure 3.16d), which would involve bonding between Si atoms of the nanosheet. In contrast, the main contribution to the conduction band is from the C 2p_x and 2p_y orbitals (Figure 3.16e). The deeper valence states, between ca. 4 and 10 eV below the highest occupied level, are primarily comprised of Si s states, with minor contributions from the C orbitals. The H s bands (Figure 3.16f) are generally coincident with the C p_x , p_y , and p_z bands, as expected from their bonding in the phenyl groups. In addition, the overlap of the H s and Si p_z states, primarily between 2 and 4 eV, indicates the formation of strong Si–H bonds, as indicated by the electron localization function (ELF) plot. Overall, the spiky profile of the CDOS indicates a weak electronic interaction between these atoms, which is also suggested by the band structure and the ELF profile.

3.5.5  Electrical properties of lithiated SiNS

Apart from the abovementioned methods, the Si-H groups present on the Si₆H₆ surface can be modified by mechanochemical solid-phase reaction (Ohashi et al. 2014). Thus, lithiated SiNS (Li-SiNS) were synthesized by mechanical kneading of Si₆H₆ with metallic lithium (Li) using an agate mortar (Figure 3.17a). This mechanochemical lithiation of Si₆H₆ proceeded at room temperature under an argon atmosphere with the release of hydrogen gas, with Li-SiNS being obtained as a homogeneous powder after kneading for 30 min (Figure 3.17b). The substitution rate of the Si-H groups with Li is controlled by the additive amount of metallic lithium. Thus, Li-SiNS having different lithium content (Si₆H_{6− n} Li _n ) can be prepared (Figure 3.17c).

Figure 3.17   (a) Schematic image of the lithiation reaction. (b) Pictures of the lithiation and mechanochemical reaction processes of Si6H6 with Li. (c) Pictures showing the color of the obtained composites.

FTIR measurements indicate that the intensity of the Si–H stretching vibration at 2100 cm⁻¹ is reduced in accordance with the rate of lithiation, while the Si–Li stretching vibration is confirmed at 450 cm⁻¹ (Figure 3.18a). XANES measurements also support the formation of Si–Li bonds. XRD measurements reveal typical diffraction peaks of Si₆H₆, with layered ((001) and (002) planes) and hexagonal crystal structures of the Si framework ((100) and (110) planes) changing, depending on the extent of the lithiation process. In addition, no diffraction peak attributed to Li metal is observed in the lithiated samples. These results reveal the distortion of the Si framework and the collapse of the layered structure upon lithiation. The electron band structure of Si₆H₆ is also changed as a result of the lithiation process. As shown in Figure 3.18b, the diffuse reflection absorption spectra of Si₆H_{6− n} Li _n reveal a shifting of the absorption band edge of Si₆H_{6− n} Li _n to longer wavelengths while showing lower band gap energies (0.85 eV) compared with Si₆H₆ (2.2 eV). The completely lithiated sample (Si₆Li₆) shows semiconductor-like conductivity values.

Figure 3.18   (a) FTIR and (b) diffuse reflection absorption spectra of layered polysilane (black) and Li-SiNS (blue: Si6H6/3Li, red: Si6H6/6Li).