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Stability and Exfoliation of Germanane: a Germanium Graphane Analogue

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Stability and Exfoliation of Germanane: a Germanium Graphane Analogue Stability and Exfoliation of Germanane: A Germanium Graphane Analogue Undergraduate Thesis Presented in Partial Fulfillment of the Requirements for the Degree of Bachelor of Science in Chemistry with Research Distinction in Chemistry in the College of Arts and Sciences of The Ohio State University By Elisabeth Bianco The Ohio State University May 2013 Thesis Committee: Joshua Goldberger, Advisor Patrick Woodward Abstract Graphene's success has shown that it is not only possible to create stable, single-atom thick sheets from a crystalline solid, but that these materials have fundamentally different properties than the parent material. We have synthesized for the first time, mm-scale crystals of a hydrogen-terminated germanium multilayered graphane analogue (germanane, GeH) from the topochemical deintercalation of CaGe2. This layered van der Waals solid is analogous to multilayered graphane (CH). The surface layer of GeH only slowly oxidizes in air over the span of 5 months, while the underlying layers are resilient to oxidation based on X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR) measurements. The GeH is thermally stable up to 75 oC, however, above this temperature amorphization and dehydrogenation begin to occur. These sheets can be mechanically exfoliated as single and few layers onto SiO2/Si surfaces. This material represents a new class of covalently terminated graphane analogues and has great potential for a wide range of optoelectronic and sensing applications, especially since theory predicts a direct band gap of 1.53 eV and an electron mobility ~five times higher than that of bulk Ge. ii Acknowledgements This work was supported in part by an allocation of computing time from the Ohio Supercomputing Center, as well as the Analytical Surface Facility at OSU chemistry, supported by National Science Foundation under the grant number (CHE- 0639163). Thank you to The Ohio State University Undergraduate Instrumental Analysis Program for the use of their instrumentation. This project was supported by the National Science Foundation under project number DMR-1201953, the Army Research Office (W911-NF-12-1-0481), the Center for Emergent Materials at The Ohio State University, a NSF MRSEC at The Ohio State University (Grant DMR-0820414), The Ohio State University Materials Research Seed Grant Program, and startup funding from The Ohio State University. A special thank you to Oscar Restrepo and Prof. Wolfgang Windl in the Materials Science and Engineering Department at Ohio State for performing the band structure simulations reported herein. Thank you to my labmates, especially Sheneve Butler and ShiShi Jiang, for your contributions to this work. Finally, my deepest gratitude is to my advisor, Prof. Josh Goldberger, for your unwavering support and mentorship. iii Vita Education B.S., Chemistry ……………………………………………..The Ohio State University Cum Laude 2013 with Research Distinction in Chemistry Publications 1. E. Bianco, S. Butler, S. Jiang, O. Restrepo, W. Windl, J. Goldberger “Stability and Exfoliation of Germanane: A Germanium Graphane Analogue” ACS Nano. DOI: 10.1021/nn4009406. iv Table of Contents Abstract ………………………………………………………………………………. ii Acknowledgements ………………………………………………………………….. iii Vita …………………………………………………………………………………... iv List of Figures ……………………………………………………………………….. vi Chapter 1: Synthesis and Characterization of Germanane 1.1: Introduction ……………………………………………………………… 1 1.2: Synthesis and Structural Characterization ………………………………. 3 Chapter 2: Air Stability ……………………………………………………………... 10 Chapter 3: Optical Properties ……………………………………………………….. 12 Chapter 4: Thermal Stability ………………………………………………………... 15 Chapter 5: Theoretical Band Structure ……………………………………………… 19 Chapter 6: Exfoliation of Single Layers ……………………………………………. 22 Conclusion ………………………………………………………………………….. 24 Methods …………………………………………………………………………….. 25 References …………………………………………………………………………... 27 v List of Figures Figure 1. a) Schematic of GeH synthesis; Images of b) CaGe2 and c) GeH crystals; Powder XRD patterns of d) CaGe2 and e) GeH …………………. 5 Figure 2. a),b) TEM micrographs of GeH; c) Electron diffraction pattern of GeH; d) Energy dispersive X-ray spectroscopy of the GeH sheets……………... 6 Figure 3. Mid-FTIR spectrum of GeD ……………………………………………….. 7 Figure 4. a) Mid- FTIR of GeH; b) Raman spectrum of GeH and Ge powder; c) XPS spectrum of the Ge 2p peak for GeH and a Ge(111) wafer ……………….. 9 Figure 5. a) Time dependent FTIR of a GeH platelet after exposure to ambient atmosphere; b) Time dependent XPS spectra of GeH after exposure to atmosphere and after Ar etching …………………………………….. 11 Figure 6. a) DRA spectrum of GeH; b) Calculated electronic band structure of an isolated single layer of GeH, and the carrier effective masses for each extrema …………………………………………………………. 13 Figure 7. Fits of the absorption spectrum of unannealled GeH to different band structures, according to Tauc/Davis-Mott expressions of 2D and 3D densities of states …………………………………………………………....... 14 Figure 8. a) TGA analysis of GeH; b) DRA spectra, c) XRD patterns, and d) Raman spectra of GeH measured after annealing treatments ……………....16 Figure 9. a) DRA and b) Raman spectra of GeH synthesized using HBr and HI after annealing treatments in 5% H2/Ar…..………………………………..18 Figure 10. Calculated electronic band structure of 2-layer GeH from a) K-Γ-M, b) H-A-L, and c) Γ-A, K-H, and M-L ………………………………... 20 Figure 11. AFM micrograph, height profile, and optical micrograph of a) few layer, and b) single layer GeH ………………………………………….... 23 vi Chapter 1: Synthesis and Characterization of Germanane 1.1 Introduction The discovery of single-layer graphene has shown that it is not only possible to create stable single-atom thick layers from anisotropic crystal structures held together mainly via weak van der Waals interactions, but that these isolated layers can have fundamentally different electronic structures and properties than the parent material.1,2 For example, in a single layer of graphene electrons behave as massless Dirac fermions, resulting in potential applications for sensors, high mobility transistors, transparent conducting electrodes, and photocatalyst supports.3-5 This has sparked much recent interest toward understanding how the bulk properties of other layered van der Waals bonded crystal structures (MoS2, WS2, Bi2Se3, BN, etc…) change when prepared as 6,7 isolated individual sheets. For example, bulk MoS2 normally has an indirect band gap 8-10 at 1.29 eV, whereas isolated single layers of MoS2 have a direct gap (1.8 eV). Single 10 layers of MoS2 have also attracted much interest as high mobility transistors. Most of the layered materials studied to date are comprised of neutral or ionic layers and lack the possibility for chemical functionalization. Designing electronically active layers that could be covalently modified without disrupting the electronically relevant state would be incredibly advantageous for a wide range of applications. The nature of this terminal substituent would potentially give a synthetic handle for not only tuning the entire electronic structure based off of its identity and electron withdrawing capability, but could also enable the grafting of functional ligands for high-specificity sensing applications. Graphene can be grafted with organic components, oxidized or 1 even terminated with hydrogen atoms to form graphane (CH);11,12 however, these modifications disrupt the excellent carrier mobility in graphene, and also are not stable long term.13 Other Group IV layered lattices, may maintain appreciable conductivity 3 2 3 when the atoms are in the sp -hybridized state. Recently, single-layer thick sp and sp group IV systems have attracted considerable theoretical and experimental interest.14-17 It has been previously shown that layered Zintl phases such as CaSi2 and CaGe2 can be topochemically deintercalated in aqueous HCl at low temperatures to produce layered silicon and germanium solids.18-20 The resultant four-coordinate puckered lattice of Si and Ge atoms has an analogous geometry to sp3-hybridized graphane, or a Si/Ge(111) surface in which every Si/Ge atom is terminated with either –H or –OH above or below the layer.18,21 There is a great propensity for the silicon lattice to oxidize, initially forming siloxene (SiH0.5(OH)0.5) sheets that are terminated with either Si–H or Si–OH bonds at the fourth coordination site, which eventually degrade to form SiO2 under ambient conditions.20,22 Appreciable Si–OH bond formation is always observed in the FTIR spectrum as an intense, broad Si–O stretching mode at 1000-1200 cm-1, even after HF treatment.20 In contrast, the air- and thermal-stability of germanane (GeH) has not been rigorously characterized. Resistance to oxidation is an essential prerequisite for many future applications. From previous work,18 there remain questions about the structure, air-stability, thermal-stability, and crystallinity of bulk GeH, as characterization did not include transmission electron microscopy, Raman spectroscopy, and XPS data. Additionally, while previous work focused on interconverting µm-thick epitaxial thin films of CaGe2 on Ge(111) into GeH, the present 2 study focuses on the synthesis of free-standing crystals of CaGe2. Furthermore, the exfoliation of single- and few-layer sheets of GeH has yet to be shown. Herein, we demonstrate for the first time, the gram-scale synthesis of mm-scale crystals of a layered GeH van der Waals solid that have platelet-like morphologies akin to Kish graphite. We
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