Inter- and Intralayer Compression of Germanane - The Journal of

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Inter- and Intralayer Compression of Germanane Yanmei Ma,†,‡ Yuanzheng Chen,‡,⊥ Yanming Ma,‡ Shishi Jiang,§ Josh Goldberger,§ Thomas Vogt,∥ and Yongjae Lee*,† †

Department of Earth System Sciences, Yonsei University, Seoul 120-749, Korea State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China § Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States ∥ NanoCenter and Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States ⊥ School of Physical Science and Technology, Key Laboratory of Advanced Technologies of Materials, Ministry of Education of China, Southwest Jiaotong University, Chengdu 610031, China ‡

ABSTRACT: The inter- and intralayer compression in germanane (GeH) were investigated at room temperature and pressures up to 10 GPa using a diamond-anvil cell (DAC) in combination with X-ray diffraction and micro-Raman spectroscopy. To examine the effect of the pressure-transmitting medium (PTM) on the interlayer compression leading to a possible insertion and exfoliation, two separate experiments were performed using water as PTM and no PTM. The results indicate that both the inter- and intralayer compression are dependent on the PMT, and the linear compressibility is 0.0274(1) and 0.0031(9) GPa−1 along the c- and a-axis based on the shifts of the (002) and (100) reflections for water PTM, and 0.0190(6) and 0.0026(9) GPa−1 along the c- and a-axis when pressurizing without PTM. High-pressure Raman spectra indicate that the intralayer Ge−Ge stretching mode is also dependent on the PMT. It exhibits a blue-shift of 3.01 and 2.66 cm−1/GPa when using water as PTM and without PTM, respectively. The equation of state determined from our experiments yields a bulk modulus of 9.6(2) GPa with B′0 = 12 for GeH. Theoretical calculations were performed to understand the pressure dependence on the electronic band structure and the observed Raman spectrum.

I. INTRODUCTION In recent years, studies on inorganic layered materials have focused on the synthesis and characterization of exfoliated monolayers or materials having only a few layers.1−8 The layered transition metal chalcogenides (MoS2, WS2, TiSe2, Bi2Se3) are among these new 2-dimensional (2D) materials, many of which have high electron and hole mobilities and maintain a band gap for up to a few layers. For example, bulk MoS2 is a well-known semiconductor with an indirect band gap of 1.29 eV, whereas its monolayer structure has a direct band gap of ∼1.80 eV4 and a quite high mobility of 200 cm2/(Vs).5 These 2D structures are advanced materials promising new device applications. Germanane, a newly synthesized, singlelayered, two-dimensional (2D) nanomaterial, attracted much attention due to the possibility of using covalent chemistry to tailor its optoelectronic and other properties.1−3,8,9 Pressure can induce direct and significant changes in atomic and electronic structures, and allows the tuning of the properties. The compression behavior of layered van der Waals solids has received considerable attention in recent years.10−14 Wen et al.10 theoretically demonstrated that insulating graphane layers can result in metallic behavior © 2014 American Chemical Society

under pressure. By inducing a metathesis reaction between ReCl5 and Li3N under high pressure of 7.7 GPa and temperature of 1873 K, the layered van der Waals solid ReN2 has recently been successfully synthesized.11 Nicolle et al.12 reported the effect of the pressure-transmitting medium (PTM) on the compression of graphene layers, and found pressuremediated n-type doping when using alcohol as a PTM. In this work, the compression behavior of two-dimensional germanane has been explored by high-pressure diffraction, Raman spectroscopy experiments, and theoretical calculations. Using a diamond-anvil cell (DAC) in combination with X-ray diffraction and micro-Raman spectroscopy, we investigated the behavior of germanane (GeH) in both hydrostatic and nonhydrostatic conditions up to 10 GPa. We obtained interand intralayer linear compressibilities based on the shifts of the (002), (100), and (101) reflections. The effect of the PTM on the pressure-induced upshift of Ge−Ge stretching mode was measured using Raman spectroscopy. Furthermore, we have Received: July 25, 2014 Revised: October 26, 2014 Published: October 29, 2014 28196

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Figure 1. (a) Schematic crystal structures of GeH. Yellow (big) and white (small) spheres represent germanium and hydrogen atoms, respectively. (b) Calculated electron localization function (ELF) of GeH. Yellow (big) and gray (small) spheres represent germanium and hydrogen atoms, respectively. (c) Calculated electronic densities of state (DOS) of GeH. (d) Calculated electronic band structures of GeH under different pressure.

carried out first-principles density functional theory (DFT) calculations within the generalized gradients approximations (GGA) and the all-electron projector-augmented wave (PAW) method to achieve a consistent understanding of our experimental findings.

patterns.17 The sample-to-detector distance and geometric parameters were calibrated using a CeO2 standard. Highpressure XRD patterns were fitted by Le Bail profile matching procedure using the GSAS EXPGUI programs.18,19 High-pressure Raman spectra were recorded using a customized micro-Raman system including a spectrograph with 0.5-m focal distance, and a CCD detector (Princeton Instruments) at Yonsei University. The excitation source used for all Raman measurements was a Newport Excelsior 532-nm laser with 1.5-m Win backscattering geometry. The laser spot was intentionally focused to ∼20 μm on the sample via a combination of the 10× beam expander and a 20× Mitutoyo objective lens to avoid any laser-induced decomposition of the sample. The spectral resolution was about 1.5 cm−1 with a SP2556 spectrograph and a Spec-10:100BR/LN CCD detector made by Princeton Instruments. The average acquisition time used for a single spectrum was about 5 s. We constructed the GeH structure from the layered Zintl phase of CaGe2 and then performed ab initio structural relaxations from 0 to 10 GPa using density functional theory (DFT) within the generalized gradient approximations (GGA)20 and the all-electron projector-augmented wave (PAW)21,22 method, as implemented in the VASP code.23 The PAW potentials were taken from the VASP library in which 4s24p2 and 1s1 are treated as valence electrons for the Ge and H atoms, respectively. The VASP method that adds a correction to the conventional Kohn−Sham DFT energy was used to perform van der Waals (vdW) corrected DFT.24 The

II. EXPERIMENTAL METHODS Germanane crystals were synthesized according to the method reported by Bianco et al.1 Briefly, mm-scale crystals of CaGe2 were prepared by sealing stoichiometric quantities of Ca and Ge in a quartz tube in an Ar-filled glovebox, sealed, annealed to 950−1050 °C, and then slowly cooled over the course of 2−10 days. These CaGe2 crystals were then topotactically converted to GeH in concentrated HCl for 5−14 days at −40 °C, yielding single crystal GeH platelets that were ∼1−4 mm long and wide and ∼5−50 μm thick. Crystalline flakes of GeH were loaded into a symmetric MaoBell-type diamond-anvil cell (DAC) together with a small ruby sphere as a pressure gauge. The pressure was determined from the frequency shift of the ruby R1 fluorescence line.15 Water was used as a pressure transmitting medium (PTM). In-situ high-pressure angle dispersive XRD experiments were carried out up to 10 GPa using an advanced microfocus rotating anode MicroMax 007 HF X-ray source (λ = 0.71073 Å) equipped with Rigaku’s imaging plate detector at Yonsei University. The average acquisition time was 600 s. The two-dimensional diffraction images were processed using the FIT2D software16 to yield one-dimensional intensity versus diffraction angle 28197

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diffraction patterns of GeH are shown in Figure 2. It can be seen that there are three main diffraction peaks from a flake

phonon calculations were performed using a plane-wave pseudopotential scheme within linear response density-functional theory, as implemented in the QUANTUM-ESPRESSO package. The local density approximation (LDA) exchangecorrelation function with the norm-conserving pseudopotentials was employed. The electronic wave functions and the electron density were expanded by the plane-wave basis sets using a cutoff energy of 80 Ry. The Raman intensities were computed from the second-order derivative of the electronic density matrix with respect to a uniform electric field. The Raman spectrum was calculated and applying a 20 cm−1 Gaussian smearing.

III. RESULTS AND DISCUSSION The crystal structure of GeH at ambient conditions is illustrated in Figure 1a. GeH is a fully hydrogenated derivative of germanene structurally similar to graphane, possessing weak van der Waals forces between the interlayers along the hexagonal c-axis direction. The arrangement of hydrogen atoms within the GeH layer favors the chairlike setting with hydrogen atoms alternating on both sides of the layer. After constructing the GeH structure from the layered Zintl phase of CaGe2, we performed ab initio structural relaxations. We found that optimizing the structure of germanane without considering any van der Waals forces, the optimized lattice parameters (a = b = 4.081 Å, c = 12.949 Å) are significantly different from the experimental lattice parameters (a = b = 3. 889 Å, c = 11.188 Å), especially along the c-axis. After considering van der Waals interactions, we performed the van der Waals-corrected DFT computations, which resulted in values closer to the experimental lattice parameters (Table 1). The electron

Figure 2. Selected experimental XRD patterns of GeH with increasing pressure up to ca. 10 GPa and then decompression to ambient pressure (from bottom to top) at room temperature in (a) no PTM and (b) water as PTM. Asterisk marked in (b) represents the peak of ice.

sample of GeH, the (002), (100), and (101) reflections, where the (100) reflection dominates. Le Bail whole profile fitting of the XRD pattern confirms a hexagonal structure with lattice parameters a = 3. 889(1) Å, c = 11.188(1) Å, and unit cell volume V0 = 146.55(1) Å3, in agreement with the previous study (Table1).1 Upon compression, all diffraction peaks shift toward higher 2θ angles and reveal no indication of a phase transition at elevated pressures. Meanwhile, one observes that the (002) peak vanishes at about 6.7(1) GPa using water as a PTM (Figure 2b), suggesting the structure loses its long-range ordering along the c-axis, possibly due to the insertion of water and a possible turbostratic disordering. The pressure-dependent lattice parameters and the unit cell volume are depicted in Figure 3. Both the lattice parameters and the unit cell volume decrease monotonically with increasing pressures. The experimental pressure−volume data of GeH were fitted to the Birch−Murnaghan (BM) equation of state (EOS)(Figure 3b):

Table 1. Experimental and Calculated Lattice Constant, Unit Cell Volume, and Equation of State Parameters (B0 and B′0) of the GeH at Ambient Pressure a

present expt present exptb calcd calcd (vdW) exptc a

a (Å)

c (Å)

V (Å3)

B0 (GPa)

B′0

3.889(1) 3.878(6) 4.081(1) 4.017(3) 3.880

11.188(1) 11.129(7) 12.949(9) 12.7551(2) 11.04

146.55(1) 145.00(1) 186.76(1) 178.41(3) 143.92

9.6(2) 6.5(2)

12 12

Runs without PTM at all. bRuns with water as PTM. cRef 1.

localization function (ELF) within the GeH structure is shown in Figure 1b, clearly indicating the existence of a strong covalent character of both the Ge−H and Ge−Ge bonds. The calculated electronic densities of state (DOS) in Figure 1c reveal that the top valence bands are essentially composed of states derived from 1s-states of H atoms and p-character of Ge atoms, while the bottom of the conduction band is formed by s states and p states of Ge. The Ge p- and H s-states are energetically degenerate at the valence band. This facilitates the Ge−H hybridization and thus the formation of directionally covalent bonds. With increasing pressure up to 20 GPa, the structure and bond characters of GeH have no obvious change. Band structure calculations suggest that germanane is a direct band gap material and its band gap is almost unchanged as pressure increases up to 20 GPa (Figure 1d). The orbital character of each band in these calculations is similar to that of studies on other group IV graphane analogues.25 High-pressure XRD measurements on GeH were performed up to 10 GPa using water as a PTM and without PTM. Selected

P = 3/2B0 [(V0/V )7/3 − (V0/V )5/3 ] {1 + 3/4(B′0 − 4) × [(V0/V )2/3 − 1]}

whereV0 is the volume per formula unit (f.u.) at ambient pressure, V is the volume per f.u. at pressure P given in GPa, B0 is the isothermal bulk modulus, and B′0 is the first pressure derivative of the bulk modulus. We obtain a B0 of 6.5(2) GPa with B′0 = 12 in water as PTM and a B0 of 9.6(2) GPa with B′0 = 12 with no PTM present. The lower B0 value when water is present as a PTM indicates weaker interaction along the c-axis when water is inserted under pressure. On the other hand, the layer compressibility, β = −δL/δP/ L0, where L0 is the layer spacing at ambient conditions, was 28198

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Figure 3. Pressure dependence of the lattice parameters (a) and cell volumes (b) of GeH. Red and black represent the runs without PTM at all and with water as PTM, respectively. Solid lines in (b) are the fitting with BM EOSs.

found to be 0.0274(1) and 0.0031(9) GPa−1 for (002) and (100) reflections in water, respectively. In the experiment without PTM, the layer compressibilities were found to be 0.0190(6) and 0.0026(9) GPa−1 for (002) and (100) reflections. We note that the linear compressibilities are highly anisotropic in GeH, i.e., the (002) interlayer compressibility (caxis) is ca. an order of magnitude higher than the (100) intralayer compressibility (a-axis) regardless of the presence or absence of PTM. The different nature in bonding in and between the layers are the major causes of this highly anisotropic compression behavior. The van der Waals forces between the layers result in an easy compression along the caxis, whereas Ge−Ge bonding within the layers is strongly covalent, and resists compression. The significant difference of the compressibility between the inter- and intralayer has been also observed in graphite.26 The interlayer modulus (Bc = 35.7 GPa) along the c-axis is much smaller than that of the intralayer along the a-axis (Ba = 1250 GPa).26 This suggests that the c-axis direction of the interlayer is more compressible than that of the intralayer for graphite. In addition, the larger bulk modulus (B0 = 33.8 GPa, B′0 = 8.9) of graphite26 indicates that the GeH is much easier to compress than graphite. Micro-Raman spectroscopy has established itself as one of the main techniques to probe 2D structured materials.14,27 Figure 4 shows the micro-Raman spectra from a flake-shaped GeH sample in water and without PTM at pressures up to 10 GPa in the region of 150−450 cm−1. The main vibrational mode in GeH appears at 301.5 cm−1 and corresponds to the E2 intralayer Ge−Ge stretching mode and is slightly blue-shifted compared to the 297 cm−1 Raman mode for crystalline germanium.1 A second low-frequency vibrational A1 mode occurs at 226.6 cm−1. The observed two modes correspond well to the band positions reported in the literature for GeH.1 With increasing pressure, the vibrational peak of the Ge−Ge stretching mode shifts toward high frequencies, which is in good agreement with our theoretical calculations for the Raman-active modes of the hexagonal phase (Figure 5). Figure 6 shows the experimental pressure dependence of the Raman phonon frequency during compression from 0 to 10 GPa. The Raman response of GeH under pressure is strongly influenced by presence or absence of the PTM. We observe larger shifts of the Ge−Ge stretching mode to higher wave numbers in water than in the absence of a PTM. The frequency increase of the

Figure 4. Experimental Raman spectra of GeH at different pressures in the 150−450 cm−1 frequency region in (a) no PTM and (b) water as PTM at room temperature.

Ge−Ge stretching mode is 3.01 cm−1/GPa in water, whereas without PTM this mode exhibits a smaller pressure dependent increase of 2.66 cm−1/GPa. We argue that this is due to the interactions between the GeH layers and water molecules. A similar effect has been observed in the Raman spectra of carbon nanotubes at high pressure.28−30 Gao et al.28 compared the high-pressure Raman spectra using five different organic solvents as PTM. Their experiments have shown that the pressure dependence of the E2g stretching mode of the C−C bonds at 1583 cm−1 (G band) is significantly larger in the presence of PTM than without. Moreover, the linear dependence of the Raman G band with pressure depends also on the molecular weight of the PTM. Merlen et al.29 have observed that the PTM influences the pressure-dependent evolution of the Raman response in nanotubes. This is possibly due to the van der Waals interactions with an adsorbed layer of PTM molecules.28−31 28199

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be ca. 0.0274(1) GPa−1 in water. The intralayer Ge−Ge stretching mode is found to be dependent on the presence of a PTM and shows a ca. 12% decrease in its frequency increase without water being present. On the basis of theoretical calculation results, we propose that the possible nature of the inter- and intralayer compressions are related to van der Waals interactions modified by inserted water molecules.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: +82-2-2123-5667. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Global Research Laboratory program of the Korean Ministry of Science, ICT and Planning (MSIP). Y.M. thanks the support from the BK21Plus Institute of Earth, Astronomy, and Atmospheric Sciences of Yonsei University, and the National Natural Science Foundation of China (Grant 11304114).

Figure 5. Simulated Raman spectra of GeH at different pressures in the 150−450 cm−1 frequency region.

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REFERENCES

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Figure 6. Experimental pressure dependence of Ge phonon frequency in GeH during compression up to 10 GPa. Red and black symbols represent the runs without PTM and with water as PTM, respectively.

This work is the first experimental report on the highpressure behavior of germanane. Previous studies have reported uniaxial strain in graphene and graphite fibers.32,33 Several authors applied hydrostatic pressure on graphite and found upshifts in the C−C stretching of 4.1−4.7 cm−1/GPa.26,34,35 Moreover, carbon nanotubes show upshifts in C−C stretching of 6.5−10 cm−1/GPa.36,37 In both theoretical and experimental studies, the van der Waals interactions have been identified as important factors contributing to the frequencies of tangential C−C stretching modes and its pressure dependence.31,38,39 On the other hand, the pressure dependent on the Ge−Ge mode in GeH using water as PTM is smaller than 3.85 cm−1/GPa obtained in crystalline germanium using methanol−ethanol mixture as PTM.40

IV. CONCLUSIONS In summary, we have investigated the inter- and intralayer compression behavior of germanane (GeH) for the first time, with and without water as a PTM in a DAC up to 10 GPa at room temperature. The interlayer linear compressibility based on the gradual shifts of the basal (002) reflection was found to 28200

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