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Improved Topotactic Reactions for Maximizing Organic Coverage of Methyl Germanane Shishi Jiang, Maxx Q. Arguilla, Nicholas D. Cultrara, and Joshua E. Goldberger Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01757 • Publication Date (Web): 28 May 2016 Downloaded from http://pubs.acs.org on June 7, 2016

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Chemistry of Materials

Improved Topotactic Reactions for Maximizing Organic Coverage of Methyl Germanane Shishi Jiang, Maxx Q. Arguilla, Nicholas D. Cultrara, Joshua E. Goldberger* Department of Chemistry and Biochemistry, The Ohio State University, Columbus, OH USA 43210 ABSTRACT: The topotactic transformation of Zintl phases such as CaGe2 into organic-terminated germanium graphane analogues using haloalkanes is a powerful route for generating new 2D optoelectronic and spintronic building blocks. However, having uniform ligand coverage is necessary for optimizing the properties and stability of these single atom-thick frameworks. Here, we compare the effectiveness of different topochemical methods to maximize methyl-termination in GeCH3. We show that a previously developed CH3I/H2O phase transfer route produces a small percentage of partially oxidized germanane. The partially oxidized termination is readily removed upon HCl treatment leading to Ge-Cl termination, but rapidly reoxidizes after exposure to the ambient atmosphere. We then show that a one-pot route with CH3I in distilled CH3CN solvent and at least six equivalents of H2O results in no oxidation. The GeCH3 prepared from this one-pot route also has an increased –CH3:–H ratio of termination from ~90:10 to ~95:5, is air-stable, has greater thermal stability, has a sharper absorption onset, and more narrow band edge photoluminescence, all of which are signatures of a less defective semiconductor.

INTRODUCTION Two-dimensional (2D) van der Waals materials have attracted considerable interest in the past couple of years in applications ranging from electronics, optoelectronics, sensing and energy conversion.1-8 One particularly intriguing family of 2D materials is the covalently terminated group 14 graphane analogues, often referred to as silicane/germanane/stannanane. The Si/Ge/Sn atoms in these graphane analogues crystallize into puckered honeycomb frameworks, in which every atom in the framework is coordinated to a 4th surface terminating ligand.9 This is structurally analogous to ligand-functionalized Si(111) or Ge(111) surfaces10. Since every atom in these 2D networks is covalently bound to a ligand, the identity of this ligand can provide a powerful synthetic handle for tuning the electronic structure and properties in these materials. For example, with the appropriate surface terminating ligand, these 2D materials can have direct and tunable band gaps,11-16 potentially enhancing silicon’s and germanium’s performance in optoelectronic semiconductor applications such as photovoltaics, photodetectors, light-emitting diodes, and lasers. Additionally, there have been many exciting theoretical predictions of 2D topological insulating states in these group 14 graphane analogues, including highly strained, methyl-terminated germanane (GeCH3).17 Furthermore, the germanane derivative have attracted interest in spintronic applications.18, 19 For example, there have been numerous calculations of ferromagnetic behavior in partially functionalized germanane frameworks, in which one side is covalently terminated with –H or –CH3, and the other side of the framework remains unfunctionalized. However, before any of these predicted applications can be realized, understanding how to control the degree and uniformity of surface functionalization in these group 14 graphane analogues is essential. We have previously established the synthesis of two different ligand-terminated germanium graphane analogues: hydrogen-terminated germanane (GeH) and methylterminated germanane (GeCH3).13, 14 These are prepared by topotactically reacting β-CaGe2, a Zintl phase that consists of single-atom thick layers of (Ge-)n frameworks covalently

bound in a puckered honeycomb arrangement with each layer separated by Ca2+ counterions, using electrophilic reactants such as aqueous HCl or CH3I. The influence of surface functionalization chemistry is significant; GeCH3 is more thermally stable than GeH by 150 oC, and also exhibits strong photoluminescence (PL) with a higher band gap by 0.1 eV.13, 14 In these studies, multilayer crystals of GeCH3 were prepared using a CH3I/H2O phase transfer route, in which CaGe2 was reacted in the CH3I layer, and the CaI2 byproduct transferred into the H2O layer. Due to the large excess of H2O, the prepared GeCH3 had residual –H termination, and the ratio of –CH3:–H coverage was estimated to be ~90:10 from elemental analysis and thermogravimetric studies. Furthermore, the air stability of GeCH3 prepared using this route is not yet reported. Since 2D materials are very sensitive to the immediate environment,15, 20-24 a small difference in the ratio of –CH3 to –H can significantly influence the stability and properties of GeCH3. Similarly, the air-stability and surface recombination velocity of Si(111) surfaces is sensitive to the density of the ligand.25 As another example, it has been shown that treating MoS2 in organic superacid dramatically increases the fluorescence quantum yield from below 1% to 95%.26 Thus, understanding how to optimize the topotactic chemistry to maximize the degree of –CH3 termination is essential for both enhancing the performance of the semiconducting properties of GeCH3, as well as enabling further studies on manipulating the electronic structure via the identity of the ligand. Here we establish a one-pot topotactic route towards GeCH3 that reduces the amount of residual Ge-H and further enhances the thermal and air stability and optical properties. First we show that the GeCH3 synthesized using the original phase transfer method contains small amounts of partially oxidized germanane due to water in the CH3I layer. Upon HCl treatment, the partially oxidized germanane is removed resulting in Ge–Cl termination at a Ge:Cl ratio of ~95:5. However, Ge-Cl re-oxidizes within one day of air exposure. Second, GeCH3 prepared using a one-pot method by reacting CaGe2, CH3I, H2O in an CH3CN solvent leads to an increase in the –CH3:-H ratio to ~95:5 with no

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apparent oxidation. At least six equivalents of water with respect to CaGe2 are needed to coordinate to the Ca2+ cation to ensure the completion of the topotactic conversion. GeCH3 prepared from this one-pot method has improved air and thermal stability, along with a sharper absorption onset, and more narrow band edge photoluminescence, all of which are signatures of a less defective semiconductor.

EXPERIMENTAL SECTION Synthesis of CaGe2. In a typical reaction, Ca and Ge were loaded into a quartz tube in a 1:2 stoichiometric ratio. The quartz tube was sealed under vacuum and annealed at 950–1,050 oC for 16–20 h and then slowly cooled down to room temperature.13 Synthesis of GeCH3. In the phase transfer method,14 the CaGe2 crystals were loaded into an extraction thimble, placed in a beaker, and fully immersed in iodomethane. Caution; Iodomethane is a known carcinogen. A distilled water layer was then added to the outside beaker, keeping care to prevent the water from entering the extraction thimble. The reaction was run at room temperature for 7 days. After the reaction, the multilayer flakes which have thicknesses on the order of 1-50 µm,14, 16 were rinsed with isopropanol and dried under vacuum at room temperature. Some of these samples were further washed with aqueous HCl followed by isopropanol rinsing, and then dried under vacuum at room temperature. In the one-pot method, CaGe2 crystals were first loaded into an air-free flask inside the glovebox. The flask was then connected to the Schlenk line, where all the joints were evacuated and filled with Ar. Liquid reagents including iodomethane, distilled acetonitrile and water were then added into the flask under flowing Ar. In a typical reaction, the molar ratios of CaGe2:CH3I:H2O were controlled at 1:30:10 while CH3CN was used as a solvent. After the reaction, the powders were washed with distilled acetonitrile inside a glovebag and then dried under vacuum on a Schlenk line. Characterization. Powder X-ray diffraction (XRD) (Rigaku MiniFlex II X-Ray diffractometer, Cu Kα radiation) was used to study the structure of GeCH3. Fourier transform infrared spectroscopy (FTIR) measurements were collected on a Perkin-Elmer Frontier Dual-Range FIR/MidIR spectrometer that was loaded in an Ar-filled glovebox. X-ray photoelectron spectroscopy (XPS) was collected using a Kratos Axis Ultra X-ray photoelectron spectrometer equipped with a monochromated (Al) X-ray gun and Ar ion (calibrated using SiO2) was used for etching. The percentage of each oxidization state was calculated by applying a standard Gaussian fit to the XPS peak. Diffuse reflectance absorption (DRA) (Perkin-Elmer Lambda950 UV/Vis Spectrometer) and photoluminescence (Cary Eclipse Fluorescence) measurements were conducted to study the optical properties of the bulk solid crystals. The reflectance measured from the DRA was converted into absorption via Kubelka-Munk function: F(R)=(1R)2/2R. In the PL measurements, the excitation wavelength was set to 380nm while the excitation and emission width were set to 20 nm and 5 nm, respectively. X-ray fluorescence measurements were performed using an Olympus DELTA hand-held X-ray fluorescence analyzer and calibrated with Ge and NaCl mixtures. Thermogravimetric

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analysis (Thermal Analysis, Q-500 thermogravimetric analyzer) was collected under flowing N2 with the rate at 10 oC/min. Elemental Analysis (Galbraith Laboratories, Inc.) of the C/H ratio was collected to determine the ratio of CH3-termination to H-termination in the products. For TGA and elemental analysis, samples were evacuated at 80 oC for three hours on a Schlenk line to get rid of any absorbed water or solvent.

RESULTS AND DISCUSSION GeCH3 was first synthesized using a phase transfer route in which CaGe2 was placed in a CH3I layer on an extraction thimble in a beaker and a water layer was poured into the beaker (Figure 1, top).14 The addition of H2O is necessary due to the limited solubility of CaI2 in CH3I. However, the starting material, CaGe2, has been previously established to react upon air and water exposure.27 Though it was completely immersed in the CH3I layer during this phase transfer reaction, a small amount of water and oxygen can always dissolve into the CH3I layer and partially oxidize the structure. Consequently, after reaction, the as-obtained GeCH3 is oxidized, as evidenced by FTIR and XPS data.

Figure 1. (top) Scheme of phase transfer and (bottom) onepot reactions. Here we depict the structure of GeOx as Ge-OH bonds for illustrative purposes only. Each colored sphere represents one element with blue for germanium, yellow for calcium, black for carbon, gray for hydrogen, red for oxygen and green for chlorine.

In the FTIR spectrum, a shoulder of the –CH3 rocking mode in the range of 800-960 cm-1 was observed (Figure 2a), indicating the presence of Ge-O bonding.28 HCl treatment was then followed to wash away residual germanium oxides (GeOx). After this HCl treatment, the Ge-O stretching shoulder disappeared, indicating that the GeOx is removed. However, since the Ge-O stretching mode overlaps with the strong –CH3 rocking at 770 cm-1, it is ambiguous to determine the presence of Ge-O vibration just based on the appearance of a shoulder peak. To separate the Ge-O stretching mode from –CH3 rocking modes and clearly demonstrate the appearance and disappearance of GeOx, we collected the same series of FTIR spectra with GeCD3 (Figure 2b). In the as-obtained GeCD3, it is obvious that germanium is partially oxidized with a broad Ge-O stretching mode between 750 and 930 cm-1. While it is difficult to conclusively identify the exact nature of the GeOx structure, the lack of an intense mode above 900 cm-1 suggests that there is no doubly bonded Ge=O species, and the presence of a Ge-OH stretch at 3000-3600 cm-1 is apparent (Figure S1). After HCl washing, the broad peaks at 750-930 cm-1


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and 3000-3600 cm-1 are both gone and replaced with a doublet which is assigned to a Ge-H2 bending mode as previous observed in GeH.13 However, after exposure to air for a day, the Ge-O stretching mode appears again in both GeCH3 and GeCD3 (Figure 2a,b).

Figure 2. (a) FTIR of GeCH3 and (b) GeCD3, and (c) Ge 2p3/2 XPS and (d) PL of GeCH3 prepared via the phase transfer route. All spectra were collected before (blue) and after HCl treatment (red), along with after HCl treatment and exposure to air for one day (green).

The surface oxidization states of the Ge atoms before and after HCl treatment were studied by XPS (Figure 2c). The as-obtained phase transfer GeCH3 shows a mixture of different oxidation states of Ge+ (1217.6 eV) and Ge2+/3+ (1219.4 eV), indicating that the surface of these flakes were partially oxidized. After HCl treatment, only a single peak was observed at 1217.5 eV, which is indicative of Ge+, confirming that the GeOx species was removed with HCl treatment. Upon exposure to air for one day, a Ge2+/3+ shoulder emerges at ∼1219.9 eV at a percentage of 35.6%. Consequently, although oxygen-free GeCH3 is obtained after HCl treatment, both the surface and the bulk of GeCH3 are not stable in air over time, which is in sharp contrast to the previous studies on methyl-terminated Ge (111) surfaces which are stable in air for several days.29, 30 The PL of GeCH3 is also very sensitive to oxidization (Figure 2d). The as-obtained partially-oxidized phase transfer GeCH3 has a PL at 1.90 eV with a shoulder at around 1.76 eV, both of which are higher than the 1.7 eV band edge of GeCH3 determined from absorption measurements.14 After HCl treatment, a single asymmetric PL peak was observed at 1.7 eV with a full-width-at-halfmaximum (FWHM) of ~250 meV. However, upon exposure to air for one day, the PL blue-shifted back and exhibited 1.87 eV and 1.76 eV luminescence. Combining with the observed changes in FTIR and XPS, we conclude that the PL of GeCH3 blue shifts when the germanium framework oxidizes. Previous studies have observed similar ~1.9 and ~1.8 eV photoluminescence in defective H2-annealed, or gamma ray irradiated GeO2.31-33 In these materials, this

luminescence arises from a single defect center having radical character, although the exact chemical structure of this defect center has not been convincingly elucidated. It is likely that this PL does not arise from other predicted defect centers that occurs in germanane, such as partial demethylation of the framework. Theoretical studies suggest that partial dehydrogenation of GeH will produce defect states below the conduction band minimum in GeH119, 34 If the same trend occurs for partial demethylation of x. Ge(CH3)1-x, the observed PL will result in a defect state at energies lower than the band gap. However, we observe the defect PL to have an energy that is higher than the band gap. The conclusive identification of the structure of these defect states requires a much more detailed study. To further understand the cause of the oxidization, we exposed the as-obtained phase transfer GeCH3 to air and water for various time periods respectively and collected the FTIR spectra (Figure S2, S3). The relative intensity of the Ge-O stretching shoulder did not change over a time of 26 days in air or 7 days in water, indicating that the Ge-CH3 bonds are inert to both air and water. Additionally when the as-obtained phase transfer GeCH3 is exposed to HCl, the XPS spectra shows the existence of a Cl 2p3/2 and 2p1/2 doublet, indicative of the introduction of Cl (Figure 3a). XRF measurements show the atomic ratio of Cl:Ge to be 5:95. To identify the Cl species in the sample, we then collected the FTIR spectra of GeCH3 before and after HCl treatment in the far IR range and found the emergence of a Ge-Cl stretching vibrational mode in the sample after HCl treatment at 375 cm-1 (Figure 3b), in close agreement with other reported Ge-Cl vibrational frequencies.35 This reactivity is similar to oxidized Ge (111) surfaces, where Clterminated Ge (111) surfaces can be prepared upon exposure to aqueous HCl.36

Figure 3. (a) Cl 2p XPS spectra and (b) FTIR in the far IR range of phase transfer GeCH3 collected before (blue) and after HCl treatment (red).

We have summarized the reaction scheme that happens with phase transfer GeCH3 in the top of Figure 1. During the topotactic deintercalation, due to the water sensitivity of CaGe2, the phase transfer reaction will produce partially oxidized GeCH3. This does not oxidize further since the GeCH3 bond is stable. HCl treatment successfully removes the partially oxidized GeOx species, resulting in Ge-Cl termination. Whether the oxidized GeOx species remains as part of the germanane framework or is etched away requires a more detailed investigation. However, the Ge-Cl termination will readily reoxidize upon exposure to air, converting back into GeOx.



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With the above conclusions in mind, we hypothesized that a synthetic route that eliminates CaGe2 exposure to air and water would produce non-oxidized, air-stable GeCH3. Additionally, to get higher methyl coverage, the hydrogen source, water, should be avoided or at least minimized. Towards these ends, we reacted CaGe2 and CH3I with distilled acetonitrile as the solvent in a one-pot container (Figure 1, bottom). Unfortunately, no reaction was observed after a week. Reactions with other methylating reagents in distilled solvents such as dimethyl carbonate in acetonitrile, and trimethyloxonium tetrafluoroborate in dichloromethane produced, respectively, no reactivity, and no methyl-termination but an amorphized product. Since the complete removal water prevents the topotactic reaction from occurring, different equivalents of water were added into the one-pot CH3I/CH3CN reaction while keeping the CaGe2 to CH3I molar ratio at 1:30. Water was found to be essential for the reaction to proceed (Table 1). With 0-2 molar equivalents of H2O to CaGe2, no reaction was observed after a week. When the amount of water was increased to 4 equivalents, some methylation occurs, based on the emergence of –CH3 vibrational modes and Ge-C stretching modes in the FTIR (Figure S4). Additionally, the XRD pattern shows the emergence of the reflection corresponding to the interlayer spacing at 8-9 degrees 2-Theta14 (Figure S5). However, the conversion is not complete even after a month based on the presence of residual CaGe2 reflections in the XRD pattern. With a larger amount of water (H2O:CaGe2 ≥ 6), the reaction was complete after a week, similar to the reaction time needed for the phase transfer route. We determined experimentally that the amount of CaI2 produced is ~5x below its solubility limit in distilled acetonitrile, thus, the solubility of CaI2 is not limiting the reaction. We hypothesize then that water is needed to first partially coordinate with the Ca2+ ion between the layers of CaGe2, before the methylation of the germanane framework occurs, as well as transport the Ca2+ from the framework into solution. At least 6 equivalents of H2O are needed for complete conversion, because once solvated, the Ca2+ ions are hydrated by 6-8 H2O molecules.37 However, when these reactions were repeated by replacing H2O with other Ca2+ chelating molecules including EDTA (ethylenediaminetetraacetic acid) and crown ether (18-crown-6), the CaGe2 still remained completely unreacted after a week. H2O : CaGe2 (molar ratio) 0

Reaction Status No reaction after a week.

2 4

Reacted to some extent, but is not complete in a month.

6

Reaction is complete in a week.

10

Table 1. Water to CaGe2 ratio versus reaction status. The FTIR, XPS, DRA and PL, were collected on the asobtained one-pot method to compare to the phase transfer material after HCl treatment (Figure 4, S6). All spectra show similar features between both samples, with no Ge-O stretching mode observed in FTIR, a single Ge+ peak appeared at 1217.5 eV in the XPS and the same absorption onset and PL peak position, confirming that GeCH3 synthe-

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sized from this one-pot method is oxide-free. Elemental analysis shows that the one-pot method has a slightly higher methyl coverage with 95±3%, compared to 90±10% for the phase transfer method (Table S1). Additionally, the percentage weight loss in TGA also indicates an increase in methyl coverage from 84% in the phase transfer methods to 95% with the one-pot method, consistent with the elemental analysis results. Higher methyl coverage is expected since considerably less of the proton source (H2O) is present in the reaction. Furthermore, one-pot GeCH3 has a narrower PL peak (FWHM = 180 meV compared to 250 meV for the phase transfer method) and a sharper absorption onset, much closer to the ideal step function of absorption predicted by the Tauc-Davis Mott expression for a 2D direct band gap semiconductor (Figure 4b). These optical measurements indicate that a 2D material with a more uniform ligand termination will result in properties expected for ideal 2D density of states. Mixtures of different ligand termination will produce local variabilities in the electronic structure. To conclude, with this one-pot method, oxygen-free GeCH3 can be obtained without HCl treatment, with a higher methyl coverage, and with a more narrow FWHM of photoluminescence.

Figure 4. (a) FTIR, (b) DRA and PL comparison between GeCH3 prepared from the phase transfer method after HCl treatment (red, dashed lines) and the one-pot method (green, solid lines).

The potential utility of GeCH3 for any functional device strongly hinges on its air and thermal stability. Previous reports have shown that methyl-terminated Ge (111) surfaces are stable in air for several days29, 30 and the bulk GeH is resistant to oxidization for at least two months13. Here, we use FTIR and XPS to probe the presence of Ge-O in the bulk and on the surface of one-pot GeCH3, respectively. After exposure to air for 50 days, no change was observed in the FTIR spectra, especially in the Ge-O stretching range at 800-1000 cm-1 (Figure 5a) thus proving that the bulk of GeCH3 resists oxidation. XPS was measured on a pristine sample and after exposure to air for 3 days, 10 days, and 10 days plus Ar etching (estimated to be 1 nm, close to a single layer), respectively (Figure S7). After exposure to air for 3 days, a Ge2+/3+ shoulder emerges at ∼1219.3 eV (28.3% Ge2+/3+), which becomes more intense after 10 days of air exposure (44.2% Ge2+/3+). However, after Ar etching, the Ge2+/3+ almost completely disappears with 19.0% remaining. Together, the XPS and FTIR suggest that only the surface of GeCH3 becomes oxidized over time while the bulk is resilient to air, similar to what was observed in GeH.13


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The thermal stability of GeCH3 is also improved with this optimized one-pot method, based on the TGA and differential thermal analysis (DTA) (Figure 5b). From the DTA, methyl desorption temperature increases from 345 oC in the phase transfer method to 420 oC in the one-pot method. The latter desorption temperature is very close to the methyl-desorption temperatures reported for CH3-terminated Ge (111), which occurs between 400-450 oC.30

Corresponding Author

*e-mail: [email protected] Funding Sources Primary funding for this research was partially provided by the Center for Emergent Materials: an NSF MRSEC under award number DMR-1420451. Partial Funding for this research was primarily provided by NSF EFRI-1433467. J.E.G. acknowledges the Camille & Henry Dreyfus Foundation for partial support.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge the Analytical Spectroscopy Laboratory, the Surface Analysis Laboratory (NSF DMR-0114098), the Biophysical Interaction and Characterization Facility, the REEL Laboratory and the Analytical Teaching Laboratory of The Ohio State University Department of Chemistry and Biochemistry. Figure 5. (a) FTIR of GeCH3 synthesized from the one-pot method after exposure to air for various time periods. (b) TGA (top, left axis) and DTA (bottom, right axis) of GeCH3 synthesized from phase transfer (red) and one-pot (green) methods, respectively.

CONCLUSION In summary, we have compared two different approaches to topochemically prepare GeCH3 from CaGe2, a CH3I/H2O phase-transfer method and a one-pot approach. The GeCH3 produced from the phase-transfer method is always partially oxidized due to the large amount of H2O added and requires HCl treatment to remove these oxides. However, HCl treatment introduces Ge-Cl bonds, which readily reoxidize upon exposure to air. A one-pot method that minimizes the amount of water in the reaction produces oxide-free GeCH3. The GeCH3 prepared using this new method not only has higher methyl coverage and narrower photoluminescence FWHM, it also exhibits greater air and thermal stability. Since a large library of commercially available organohalides exists, this optimized route can be extended to attach different organic ligands to germanane, to allow for a systematic understanding of the effects of ligand identity on material properties. Improving the topotactic chemistry to maximize the degree of organic ligand coverage in these group 14 graphanes, is a necessary step to realize the exciting topological and spintronic applications that have been predicted for these materials.

ASSOCIATED CONTENT Supporting Information. FTIR of GeCH3 synthesized from the phase transfer method, highlighting –OH vibration; FTIR of asobtained phase transfer GeCH3 after exposure to air or water for various time periods, respectively; XRD and FTIR of samples from a typical incomplete one-pot reaction with four equivalents of water added; Ge 2p3/2 XPS spectra of GeCH3 synthesized from two different methods; Ge 2p3/2 XPS spectra of one-pot GeCH3 after exposure to air for various time periods. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

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