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Tailoring the Electronic Structure of Covalently Functionalized Germanane via the Interplay of Ligand Strain and Electronegativity Shishi Jiang, Kevin Krymowski, Thaddeus Asel, Maxx Q. Arguilla, Nicholas D. Cultrara, Eric Yanchenko, Xiao Yang, Leonard J. Brillson, Wolfgang Windl, and Joshua E. Goldberger Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04309 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 23, 2016
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Chemistry of Materials
Tailoring the Electronic Structure of Covalently Functionalized Germanane via the Interplay of Ligand Strain and Electronegativity Shishi Jiang,1 Kevin Krymowski,2 Thaddeus Asel,3 Maxx Q. Arguilla,1 Nicholas D. Cultrara,1 Eric Yanchenko,3 Xiao Yang,4 Leonard J. Brillson,3,4 Wolfgang Windl,2 Joshua E. Goldberger1* 1Department
of Chemistry and Biochemistry, The Ohio State University, Columbus, OH USA 43210 of Materials Science and Engineering, The Ohio State University, Columbus, OH USA 43210 3Department of Physics, The Ohio State University, Columbus, OH USA 43210 4Department of Electrical and Computer Engineering, The Ohio State University, Columbus, OH USA 43210 2Department
ABSTRACT: The covalent functionalization of 2D crystals is an emerging route for tailoring the electronic structure and generating novel phenomena. Understanding the influence of ligand chemistry will enable the rational tailoring of their properties. Through the synthesis of numerous ligand-functionalized germanane crystals, we establish the role of ligand size and electronegativity on functionalization density, framework structure, and electronic structure. Nearly uniform termination only occurs with small ligands. Ligands that are too sterically bulky will lead to partial hydrogen-termination of the framework. With a homogeneous distribution of different ligands, the band gaps and Raman shifts are dictated by their relative stoichiometry in a pseudo-linear fashion similar to Vegard’s law. Larger and more electronegative ligands expand the germanane framework, thereby lowering the band gap and Raman shift. Simply by changing the identity of the organic ligand, the band gap can be tuned by ~15%, highlighting the power of functionalization chemistry to manipulate the properties of single-atom thick materials.
INTRODUCTION Two-dimensional (2D) materials have generated considerable interest over the past decade because of the new physical phenomena that emerge in these atomically thin frameworks, as well as their potential for impacting diverse sets of technologies1-5. One major lesson from this work is that the surrounding environment, the substrate flatness, and the local dielectric constant all significantly influence the properties and reactivity of these single/few atom-thick materials6-12. Establishing 2D crystals in which the framework and immediate chemical environment can be modified via covalent functionalization gives a powerful handle towards designing novel properties. Indeed, there have been numerous predictions of exciting phenomena occurring in 2D crystals functionalized with specific ligands. For example, a 2D topological phase having room temperature quantum spin Hall (QSH) behavior is predicted to occur in tin graphane analogues, when the tin lattice is terminated with halogen or hydroxyl ligands, but not hydrogen.13 QSH behavior has also been predicted in iodine-terminated germanane or methyl-terminated germanane (GeCH3) having 12% tensile strain.14,15 As another example, hydrogen-terminated silicane is predicted to have an indirect gap at 2.9 eV, while termination with different ligands converts it into a material with a direct gap
ranging from 2.1–2.5 eV.16 As the covalent functionalization of numerous families of 2D materials including the group 14 graphane analogues,17-21 transition metal dichalcogenides,22-24 phosphorene,25 and MXenes,26-28 is starting to be established, systematically understanding how and to what extent the electronic structure can be tailored by the identity of the ligand is essential to effectively utilize functionalization chemistry to rationally tailor properties. Germanane, a germanium graphane analogue, is an ideal model system for probing the effects of ligand size and electronegativity on the electronic structure of a functionalized 2D framework. In contrast to other 2D materials, covalent surface termination on every atom in the framework is thermodynamically preferred. In this single atomthick puckered honeycomb framework, covalently bonded ligands will directly couple to the orbitals comprising conduction and valence bands, facilitating the tuning of electronic structure.13,16,29 Furthermore, the arrangement of Ge atoms in these 2D frameworks is identical to that of a Ge(111) surface, which has a well-established history of surface functionalization chemistry for comparison.30-32 We have previously developed a general route for creating multilayered crystals of organic-terminated germananes via the topotactic deintercalation of layered precursor CaGe2 phases with organohalides.18,33 These bulk, multilayered crystals greatly facilitate characterization of ele-
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mental composition, structure, and properties using conventional solid-state methods, in contrast to exfoliated layers. Here, we have synthesized, characterized, and modeled ten different ligand functionalized germananes in bulk powder form to establish how ligand identity influences functionalization density, and vibrational and electronic structure. Uniform ligand functionalization is only achieved with small ligands having minimal steric crowding, such as –CH2OCH3, –CH2CH=CH2, and –CH3, whereas bulkier substituents such as –CH2CH3 will result in partial – H termination. When germanane is terminated with mixtures of two different functional groups, specifically, –H and –CH3, the structure, Raman shifts, and band gaps vary almost linearly with composition based on their relative ratio, in a fashion analogous to Vegard’s Law,34 on account of the bond length changes resulting from varying functionalization. Solely by changing the identity of the ligand, germanane’s band gap can be broadly tuned by 15% from 1.45 eV to 1.66 eV. Larger and more electron-withdrawing ligands will lengthen the Ge-Ge bond and flatten the Ge-GeGe network, resulting in lower band gaps and Raman shifts. In total, this work shows the power of surface functionalization chemistry to rationally tune the structure and properties of 2D materials.
EXPERIMENTAL SECTION To synthesize CaGe2 crystals, 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 950-1050 oC for 16-20h and then slowly cooled down to room temperature.17 To synthesize various organic ligand terminated germanane (GeRxH1-x), the CaGe2 crystals were loaded into an air-free flask inside the glovebox. The flask was then connected to a Schlenk line, where all the joints were evacuated and refilled with Ar. Liquid reagents including organohalides, distilled acetonitrile and Milli-Q water were then injected into the flask under flowing Ar. In a typical reaction, the molar ratios of CaGe2/RX/H2O were controlled at 1:30:10, where CH3CN was used as a solvent.33 The reactions were running at room temperature from one to two weeks, depending on the ligand. After reaction, samples were washed with acetonitrile in a glovebag, then concentrated HCl (aq) followed by acetonitrile or dichloromethane. Afterwards, samples were dried on a Schlenk line at room temperature. To synthesize Ge(CH3)1-xHx (0 GeCH2CH=CH2 > GeCH2OCH3 (Figure 1d, Table S5). This further indicates that the E2 Raman frequency is correlated directly to the bond length and bond angle, but not the a-parameter. Many other ligand-functionalized germanane derivatives were prepared to further probe the effect of ligand sterics on the functionalization density. When functionalized with primary alkyl ligands that are -CH2CH3 or larger, the aparameter decreases with increasing chain length, and decreases even further with the bulkier secondary-butyl ligand (Figure 2a). This shrinking of the a-parameter results from partial hydrogenation of the framework and is confirmed by the presence of Ge-H stretching modes in the FTIR at around 2000 cm-1 (Figure 2b, Figure S4). From elemental analysis measurements of the C:H ratio, the de-
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gree of organic functionalization of GeRxH1-x, ranges from x=0.14(2)-0.69(9) (Table S1). This partial organic functionalization is consistent with previous on functionalized Ge(111) and Si(111) surfaces, where complete functionalization of the atop Si/Ge atoms occurs only with –CH3, C≡CH, and –C≡CCH3.32,41-43 The internuclear distance between the atop Si and Ge sites are 3.8 Å and 4.0 Å, respectively, for unreconstructed (111) surfaces, whereas these three ligands have van der Waals radii in the in-plane direction that are equal to or less than methyl, which is 2.0 Å44. Partial hydrogen-termination on these (111) surfaces occurs with bulkier ligands due to the close non-bonding H---H interaction that occurs between neighboring ligands.45 To understand why complete surface ligand termination occurs with a ligand such as -CH2OCH3 but incomplete coverage is observed with –CH2CH3, we used DFT to determine if the nearest distance between atoms on neighboring ligands for fully functionalized structures had significant strain. Molecular dynamics simulations on vacuumseparated 3x3 germanane unit cells were performed to vary the ligand conformation away from any false local minima, followed by energy minimization of the structure. For dimethyl ether (-αCH2OƔCH3), the closest atomic distance between neighboring ligands corresponds to the distance between the O and the H-bonded to the αC, which is 2.36(15) Å (Figure 2c). For ethyl (-αCH2βCH3), the distance between the hydrogen atoms bonded to the αC and βC of neighboring ligands is calculated to be much smaller at 2.09(5) Å.
Figure 2. a, Powder XRD patterns of different alkyl- and Hterminated germananes. The starred peak corresponds to the (111) reflections of an internal Ge standard. The dotted line highlights the change in (100) reflections. b, FTIR of GeCH3 (red), GeCH2CH3 (magenta) and Ge(CH2)3CH3 (cyan). Side view of c, GeCH2OCH3 and d, GeCH2CH3 with 100% organic ligand coverage, highlighting the nearest-neighbor nonbonding distances. These numbers were obtained from the average and standard deviation of the nearest interligand nonbonding distances calculated in a 3 x 3 unit cell. Each colored sphere represents one element with blue for germanium, black for carbon, gray for hydrogen and red for oxygen. e, Strain energy
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normalized per organic ligand for GeCH2OCH3 and GeCH2CH3 as a function of organic ligand coverage from DFT calculations. The germanium atoms not terminated with –CH2OCH3 or –CH2CH3 were terminated with –H. The reference is a 3 x 3 unit cell where 1/9 of the germanium atoms are functionalized with the organic ligand, with the remaining atoms –H terminated.
To further verify that the ligand sterics of –CH2CH3 prevent complete functionalization on these frameworks, the average strain energy per organic ligand was calculated for frameworks functionalized at various ligand-to-hydrogen ratios (Figure 2e).45 We first assumed that the framework is unstrained when 1/9 of the germanium atoms in a 3x3 cell are functionalized with the organic ligand, with the remaining germanium atoms –H terminated. The increase in overall energy normalized per added ligand reflects the additional strain energy of the framework. The energetic cost of replacing a –H ligand with a –CH2OCH3 ligand is ~0.15 eV across all ligand coverage densities. In contrast, with –CH2CH3, this strain energy per ligand is also ~0.15 eV below 44% coverage but increases to ~0.3 eV per ligand from 44–78% coverage. This indicates that the experimentally observed maximum 69% -CH2CH3 coverage is a consequence of the large strain energy at high coverages. In total, complete coverage can be experimentally achieved only for ligands with a small enough substituent at the βposition.
Figure 3. a, Powder XRD patterns of Ge(CH3)xH1-x with the dotted line highlighting the change in (100) reflections. The starred peak is the (111) reflection from an internal Ge standard. b, a- and c-lattice parameters of Ge(CH3)xH1-x as a func-
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tion of -CH3 ligand density. c, Raman of Ge(CH3)xH1-x samples with the dotted line highlighting the change in the frequency of E2 Raman mode. d, E2 Raman shift of Ge(CH3)xH1-x as a function of -CH3 ligand density. The error bars in d were obtained from the standard deviation of peak positions from ten different spectra for each Ge(CH3)xH1-x. e, DRA spectra of Ge(CH3)xH1-x with the intersections of two dotted lines highlighting the absorption onsets. f, Absorption onset of Ge(CH3)xH1-x as a function of -CH3 ligand density.
With larger ligands, partial hydrogenation of the germanane framework is unavoidable. Therefore to understand how homogeneous mixtures of ligand terminations affect the structure and properties of the framework, we synthesized Ge(CH3)xH1-x (0 – CH2OCH3 > –CH3 > –H (Figure 4a). The ligand electronegativity can be estimated by the field/inductive component of the Hammett constants46. From greater to smaller ligand electronegativity, the Hammett constant decreases from –CH2OCH3 > –H > –CH3 > –CH2CH=CH2 (Figure 4b). Experimentally, the observed band gap value increases from CH2OCH3 < –CH2CH=CH2 < –H < –CH3 (Figure 4c). The
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band gap increases with decreasing ligand electronegativity, with the exception of GeCH2CH=CH2. The allyl substituent is the most sterically bulky ligand of these four, and consequently strains the framework with complete termination, resulting in a lower band gap than GeH. The observed trend in band gap is consistent with the changes in the E2 Raman mode (Figure 1d). Again, DFT calculations of Ge(CH3)xH1-x layers show that the band gap also scales linearly with Ge-Ge bond length (which again is the dominating term) and Ge-Ge-Ge bond angle (Figure S11), but with a still small, but somewhat stronger influence on the Bader charge. The influence of ligand identity and band gap is true for ligand chemistries that produce partially functionalized germanane lattices. The E2 Raman shift vs. the band gap determined via absorption onset of 10 different ligandfunctionalized frameworks are plotted in Figure 5a. Generally, alkyl-functionalized germananes have band gaps that are larger than GeH, due to their electron-donating nature. GeCH3 has the highest energy band gap and E2 Raman mode among these alkyl ligands, as partial –H termination limits the total electron-donating influence of surface functionalization. With electron-withdrawing ligands such as -CH2I, and –CH2OCH3, the band gap of the functionalized germanane is lower than GeH. Ligands that have low coverage densities (GeRxH1-x; x
