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Controlled growth of ordered monolayers of N-heterocyclic carbenes on silicon

Abstract

N-Heterocyclic carbenes (NHCs) are promising modifiers and anchors for surface functionalization and offer some advantages over thiol-based systems. Because of their strong binding affinity and high electron donation, NHCs can dramatically change the properties of the surfaces to which they are bonded. Highly ordered NHC monolayers have so far been limited to metal surfaces. Silicon, however, remains the element of choice in semiconductor devices and its modification is therefore of utmost importance for electronic industries. Here, a comprehensive study on the adsorption of NHCs on silicon is presented. We find covalently bound NHC molecules in an upright adsorption geometry and demonstrate the formation of highly ordered monolayers exhibiting good thermal stability and strong work function reductions. The structure and ordering of the monolayers is controlled by the substrate geometry and reactivity and in particular by the NHC side groups. These findings pave the way towards a tailor-made organic functionalization of silicon surfaces and, thanks to the high modularity of NHCs, new electronic and optoelectronic applications.

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Fig. 1: NHCs as surface modifiers.
Fig. 2: IPr adsorption and monolayer formation on Si(111)-B.
Fig. 3: IMe adsorption on Si(111)-B.
Fig. 4: Potential energy surfaces for NHCs on Si(111)-B.

Data availability

Data that support the findings of this study are available within the paper and its Supplementary Information. Optimized geometries of isolated molecules and adlayer systems (shown in Extended Data Figs. 4 and 6 and Supplementary Fig. 5) are provided in the Supplementary Data as a set of QUANTUM ESPRESSO input files and pseudopotential files. These include all necessary coordinates, convergence parameters and non-default settings for reproducing the results in the paper. Defaults are listed in the PWscf manual (see, for example, https://www.quantum-espresso.org/Doc/INPUT_PW.html, retrieved 30 March 2021). Geometries obtained using different k-point sets, metallic smearing and dipole corrections are available on request from the corresponding authors. The calculated core-level shifts are provided in the Supplementary Information. Further requests can be directed to the corresponding authors. Source data are provided with this paper.

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Acknowledgements

We acknowledge generous financial support from the Deutsche Forschungsgemeinschaft (Leibniz Award and SFB 858 to F.G.; SCHM 1361/25 and SCHM 1361/26 to W.G.S.). N.E. acknowledges financial support from the Ministerium für Kultur und Wissenschaft des Landes Nordrhein-Westfalen, Der Regierende Bürgermeister von Berlin-Senatskanzlei Wissenschaft und Forschung and the Bundesministerium für Bildung und Forschung. C.H and W.G.S. acknowledge CINECA under the ISCRA initiative, the Paderborn PC2 Centre and HLRS Stuttgart for grants of high-performance computer time.

Author information

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Authors

Contributions

M. Dähne, N.E. and F.G. initiated the project. M. Dähne, N.E., F.G., W.G.S., S.C. and C.H. designed the experiments and calculations and coordinated the study. M.K., M. Freitag and M. Das synthesized and selected the molecules. M. Franz, S.C., D.L. and A.K.H. performed the STM and LEED measurements. S.C., R.Z., M.R. and M. Franz carried out the XPS experiments. C.H., H.A. and U.G. performed DFT calculations. M. Dähne, N.E., F.G., W.G.S., C.H., M. Franz, S.C., M.K., R.Z., H.A., M. Freitag and D.L. interpreted data. M. Franz, C.H. and H.A. wrote the manuscript, with contributions from all authors. All the authors read and commented on the manuscript.

Corresponding authors

Correspondence to Frank Glorius or Norbert Esser or Mario Dähne.

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The authors declare no competing interests.

Additional information

Peer review informationNature Chemistry thanks Xavier Roy, Aleksandr Zhukhovitskiy and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Synthesis of NHC  CO2 adducts.

a, Synthesis of IPr  CO2. b, Synthesis of IMe  CO2.

Extended Data Fig. 2 Comparison between simulated and experimental STM images of NHCs on Si(111)-B.

a,b, A single IPr molecule on Si(111)-B (VT = − 2.0 V) and (c,d) the ordered IPr monolayer (VT = + 2.8 V). e-g, A single IMe molecule on Si(111)-B (VT = + 2.0 V). f, The result of averaging the STM image of a single molecule over several rotated orientations, yielding a better agreement with experiment. h,i, A \(2\sqrt{3}\times \sqrt{3}\) domain of IMe (VT = − 2.5 V). Dashed parallelograms indicate the \(\sqrt{3}\times \sqrt{3}\) (yellow), \(2\sqrt{3}\times 2\sqrt{3}\) (magenta), and \(2\sqrt{3}\times \sqrt{3}\) (red) unit cells.

Extended Data Fig. 3 Charge density difference plots for NHCs on Si(111)-B.

a,b, Isolated IPr and IMe molecules adsorbed on a typical Si adatom. c, IMe adsorbed on a Si-Si(S5) defect. Isosurfaces are shown at (0.0025e/bohr3) of \(\Delta \rho ={\rho }^{[{\rm{Si}}(111)-{\rm{B}}+{\rm{NHC}}]}-{\rho }^{{\rm{Si}}(111)-\rm{B}}-{\rho }^{{\rm{NHC}}}\). The left of each panel illustrates the laterally integrated charge as a function of vertical position. Charge transfer magnitudes are reported in Supplementary Table I.

Extended Data Fig. 4 Alternative geometries for single IMe adsorption on Si(111)-B.

a, Experimental image of a single IMe molecule (VT = + 2.0 V). b-f, Structures computed with DFT with adsorption energies (in eV per IMe molecule) and simulated STM images. b, Upright geometry, as also shown in Supplementary Fig. 4. c, Two IMe molecules anchored to the same Si adatom (IMe-Si-IMe dimeric complex). d, An IMe-IMe dimer (strongly buckled) and singly bound to the Si adatom. e, Hydrogen co-adsorption, whereby both the IMe and H bind directly to the adatom. f, The imidazolium cation (corresponding to protonated NHC) physisorbed to the surface via the π-system. Note that adsorption energies for (e) and (f) are computed by subtracting the energy of 1/2 H2, and are not directly comparable with (b-d). STM images are computed at a bias of +2.0 V. These simulations used a \(3\sqrt{3}\times 3\sqrt{3}\) unit cell, a broadening of 0.14 eV, and Γ point sampling. Corresponding STM height profiles and XPS C1s spectra are shown in Supplementary Figs. 10 and 11, respectively.

Extended Data Fig. 5 IPr on the Si(111)7 × 7 surface.

a, STM image of the clean Si(111)7 × 7 surface (VT = − 1.5 V; IT = 100 pA). The unit cell is indicated. The red triangle thereby marks the faulted half unit cell, while the grey one marks the unfaulted half unit cell. b, STM overview image of a low IPr coverage on Si(111)7 × 7 (VT = + 2.0 V; IT = 30 pA). c, STM overview image of a high IPr coverage on Si(111)7 × 7 (VT = − 2.5 V; IT = 20 pA). The inset (size: 20 nm × 20 nm) shows an enlarged view of the area indicated by the white box, demonstrating the disorder of the film.

Extended Data Fig. 6 Formation energies of freestanding (infinite) NHC sheets as a function of intermolecular separation.

Dashed lines indicate vdW contributions. Molecules are fixed at the gas-phase geometry, but rotated according to the adsorbed geometry at full coverage, as determined from the calculations (Supplementary Fig. 5). IPr2D and IMe2D are arranged on a hexagonal grid; IMe1D corresponds to a face-on geometry, as shown. The Si(111)-B \(\sqrt{3}\)a lattice spacings are indicated. The IPr2D data show an attractive potential well (due to strong vdW interactions) in coincidence with the spacing of the \(2\sqrt{3}\times 2\sqrt{3}\) Si(111)-B lattice. This indicates that IPr molecules moving across the surface are energetically favoured to assemble at specific sites near adsorbed molecules as part of a commensurate island or domain (epitaxial growth). The IMe2D curve instead is repulsive at a molecular spacing of \(\sqrt{3}\ a\) (the minimum adatom-adatom distance), suggesting that larger spacings may be more favoured for IMe. Actually, the face-on alignment represented by IMe1D indicates that a close packing (up to ~0.4 nm) is possible along one specific direction. However, the minimum intermolecular distance possible within the adsorbed layer is determined by the spacing of adatom sites (that is \(\sqrt{3}\ a\)), as they yield the largest adsorption energy. Thus, local assembly of IMe molecules near an adsorbed molecule or island is expected to be anisotropic. In an optimally packed configuration, IMe molecules align face-on at a \(\sqrt{3}\ a\) distance, with larger spacings in other directions due to steric repulsion of side groups (Supplementary Fig. 5). This configuration is consistent with the observation in STM of small \(2\sqrt{3}\times \sqrt{3}\) islands.

Extended Data Fig. 7 Work function changes.

a, Measured secondary electron onsets for the clean Si(111)-B surface (green), for the ordered \(2\sqrt{3}\times 2\sqrt{3}\) IPr monolayer (black), and for the IMe monolayer (red). The dashed grey lines show linear fits to the data used to determine the secondary electron onsets. b, Calculated work functions and c, work function changes as a function of NHC coverage.

Supplementary information

Supplementary Information

Supplementary Figs. 1–13 and Tables 1–3.

Supplementary Video 1

The diffusion of IPr along the minimum energy path marked in Fig. 4a. The video was composed by taking seven side-view snapshots along the IPr path. Top views are also shown to illustrate the in-plane rotation.

Supplementary Video 2

The diffusion of IMe along the minimum energy path marked in Fig. 4b. The video was composed by taking 19 side-view snapshots along the IMe path.

Supplementary Data 1

Optimized geometries of isolated molecules and adlayer systems in the form of complete QUANTUM ESPRESSO PWscf input files and pseudopotentials.

Supplementary Data 2

Calculated C1s core-level shifts for the structures shown in the main text and the Supplementary Information.

Source data

Source Data Fig. 2

Raw data of the XPS spectra shown in Fig. 2.

Source Data Fig. 3

Raw data of the XPS spectra shown in Fig. 3.

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Franz, M., Chandola, S., Koy, M. et al. Controlled growth of ordered monolayers of N-heterocyclic carbenes on silicon. Nat. Chem. (2021). https://doi.org/10.1038/s41557-021-00721-2

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