Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

# 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.

## Access options

from\$8.99

All prices are NET prices.

## 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.

## References

1. 1.

Arduengo, A. J., Harlow, R. L. & Kline, M. A stable crystalline carbene. J. Am. Chem. Soc. 113, 361–363 (1991).

2. 2.

Hopkinson, M. N., Richter, C., Schedler, M. & Glorius, F. An overview of N-heterocyclic carbenes. Nature 510, 485–496 (2014).

3. 3.

Hurst, E. C., Wilson, K., Fairlamb, I. J. S. & Chechik, V. N-Heterocyclic carbene coated metal nanoparticles. New J. Chem. 33, 1837–1840 (2009).

4. 4.

Vignolle, J. & Tilley, T. D. N-Heterocyclic carbene-stabilized gold nanoparticles and their assembly into 3D superlattices. Chem. Commun., 7230–7232 (2009).

5. 5.

Ranganath, K. V. S., Kloesges, J., Schäfer, A. H. & Glorius, F. Asymmetric nanocatalysis: N-heterocyclic carbenes as chiral modifiers of Fe3O4/Pd nanoparticles. Angew. Chem. Int. Ed. 49, 7786–7789 (2010).

6. 6.

Lara, P. et al. Ruthenium nanoparticles stabilized by N-heterocyclic carbenes: ligand location and influence on reactivity. Angew. Chem. Int. Ed. 50, 12080–12084 (2011).

7. 7.

Crudden, C. M. et al. Ultra stable self-assembled monolayers of N-heterocyclic carbenes on gold. Nat. Chem. 6, 409–414 (2014).

8. 8.

Crudden, C. M. et al. Simple direct formation of self-assembled N-heterocyclic carbene monolayers on gold and their application in biosensing. Nat. Commun. 7, 12654 (2016).

9. 9.

Zhukhovitskiy, A. V., MacLeod, M. J. & Johnson, J. A. Carbene ligands in surface chemistry: from stabilization of discrete elemental allotropes to modification of nanoscale and bulk substrates. Chem. Rev. 115, 11503–11532 (2015).

10. 10.

Wang, G. et al. Ballbot-type motion of N-heterocyclic carbenes on gold surfaces. Nat. Chem. 9, 152–156 (2017).

11. 11.

Jiang, L. et al. N-Heterocyclic carbenes on close-packed coinage metal surfaces: bis-carbene metal adatom bonding scheme of monolayer films on Au, Ag and Cu. Chem. Sci. 8, 8301–8308 (2017).

12. 12.

Chang, K., Chen, J. G., Lu, Q. & Cheng, M.-J. Quantum mechanical study of N-heterocyclic carbene adsorption on Au surfaces. J. Phys. Chem. A 121, 2674–2682 (2017).

13. 13.

Larrea, C. R. et al. N-Heterocyclic carbene self-assembled monolayers on copper and gold: dramatic effect of wingtip groups on binding, orientation and assembly. ChemPhysChem 18, 3536–3539 (2017).

14. 14.

Bakker, A. et al. Elucidating the binding modes of N-heterocyclic carbenes on a gold surface. J. Am. Chem. Soc. 140, 11889–11892 (2018).

15. 15.

Lovat, G. et al. Determination of the structure and geometry of N-heterocyclic carbenes on Au(111) using high-resolution spectroscopy. Chem. Sci. 10, 930–935 (2019).

16. 16.

Smith, C. A. et al. N-Heterocyclic carbenes in materials chemistry. Chem. Rev. 119, 4986–5056 (2019).

17. 17.

Bakker, A. et al. An electron-rich cyclic (alkyl)(amino)carbene on Au(111), Ag(111) and Cu(111) surfaces. Angew. Chem. Int. Ed. 59, 13643–13646 (2020).

18. 18.

Amirjalayer, S., Bakker, A., Freitag, M., Glorius, F. & Fuchs, H. Cooperation of N-heterocyclic carbenes on a gold surface. Angew. Chem. Int. Ed. 59, 21230–21235 (2020).

19. 19.

Inayeh, A. et al. Self-assembly of N-heterocyclic carbenes on Au(111). Preprint at https://doi.org/10.26434/chemrxiv.12551627.v1 (2020).

20. 20.

Krzykawska, A., Wróbel, M., Kozieł, K. & Cyganik, P. N-Heterocyclic carbenes for the self-assembly of thin and highly insulating monolayers with high quality and stability. ACS Nano 14, 6043–6057 (2020).

21. 21.

Koy, M., Bellotti, P., Das, M. & Glorius, F. N-Heterocyclic carbenes as tunable ligands for catalytic metal surfaces. Nat. Catal. 4, 352–363 (2021).

22. 22.

Inkpen, M. S. et al. Non-chemisorbed gold-sulfur binding prevails in self-assembled monolayers. Nat. Chem. 11, 351–358 (2019).

23. 23.

Zhukhovitskiy, A. V., Mavros, M. G., Van Voorhis, T. & Johnson, J. A. Addressable carbene anchors for gold surfaces. J. Am. Chem. Soc. 135, 7418–7421 (2013).

24. 24.

Zhukhovitskiy, A. V. et al. Reactions of persistent carbenes with hydrogen-terminated silicon surfaces. J. Am. Chem. Soc. 138, 8639–8652 (2016).

25. 25.

Kim, H. K. et al. Reduction of the work function of gold by N-heterocyclic carbenes. Chem. Mater. 29, 3403–3411 (2017).

26. 26.

Lv, A. et al. N-Heterocyclic-carbene-treated gold surfaces in pentacene organic field-effect transistors: improved stability and contact at the interface. Angew. Chem. Int. Ed. 57, 4792–4796 (2018).

27. 27.

Nguyen, D. T. et al. Versatile micropatterns of N-heterocyclic carbenes on gold surfaces: increased thermal and pattern stability with enhanced conductivity. Angew. Chem. Int. Ed. 57, 11465–11469 (2018).

28. 28.

She, Z. et al. N-Heterocyclic carbene and thiol micropatterns enable the selective deposition and transfer of copper films. Chem. Commun. 56, 1275–1278 (2020).

29. 29.

Aslanov, L. A. et al. Stabilization of silicon nanoparticles by carbenes. Russ. J. Coord. Chem. 36, 330–332 (2010).

30. 30.

Stephens, L. et al. The structural and electrochemical effects of N-heterocyclic carbene monolayers on magnesium. J. Electrochem. Soc. 165, G139–G145 (2018).

31. 31.

Bent, S. F. Attaching organic layers to semiconductor surfaces. J. Phys. Chem. B 106, 2830–2842 (2002).

32. 32.

Bent, S. F. Heads or tails: which is more important in molecular self-assembly? ACS Nano 1, 10–12 (2007).

33. 33.

MacQueen, R. W. et al. Crystalline silicon solar cells with tetracene interlayers: the path to silicon-singlet fission heterojunction devices. Mater. Horiz. 5, 1065–1075 (2018).

34. 34.

Headrick, R. L., Robinson, I. K., Vlieg, E. & Feldman, L. C. Structure determination of the Si(111):B($$\sqrt{3}\times \sqrt{3}$$R30°) surface: subsurface substitutional doping. Phys. Rev. Lett. 63, 1253–1256 (1989).

35. 35.

Bedrossian, P. et al. Surface doping and stabilization of Si(111) with boron. Phys. Rev. Lett. 63, 1257–1260 (1989).

36. 36.

Baris, B. et al. Robust and open tailored supramolecular networks controlled by the template effect of a silicon surface. Angew. Chem. Int. Ed. 50, 4094–4098 (2011).

37. 37.

Wagner, S. R., Lunt, R. R. & Zhang, P. Anisotropic crystalline organic step-flow growth on deactivated Si surfaces. Phys. Rev. Lett. 110, 086107 (2013).

38. 38.

Wagner, S. R. et al. Growth of metal phthalocyanine on deactivated semiconducting surfaces steered by selective orbital coupling. Phys. Rev. Lett. 115, 096101 (2015).

39. 39.

Makoudi, Y. et al. Supramolecular self-assembly on the B-Si(111)-($$\sqrt{3}\times \sqrt{3}$$) R30° surface: from single molecules to multicomponent networks. Surf. Sci. Rep. 72, 316–349 (2017).

40. 40.

Lindner, S. et al. Arrangement and electronic properties of cobalt phthalocyanine molecules on B-Si(111)-$$\sqrt{3}\times \sqrt{3}$$ R30°. Phys. Rev. B 100, 245301 (2019).

41. 41.

Aldahhak, H. et al. Electronic structure of the $${\rm{Si}}(111)\sqrt{3}\times \sqrt{3}{\rm{R}}{30}^{\circ }-{\rm{B}}$$ surface from theory and photoemission spectroscopy. Phys. Rev. B 103, 035303 (2021).

42. 42.

Takayanagi, K., Tanishiro, Y., Takahashi, M. & Takahashi, S. Structural analysis of Si(111)-7 × 7 by UHV-transmission electron diffraction and microscopy. J. Vac. Sci. Technol. A 3, 1502 (1985).

43. 43.

de Boer, B., Hadipour, A., Mandoc, M. M., van Woudenbergh, T. & Blom, P. W. M. Tuning of metal work functions with self-assembled monolayers. Adv. Mater. 17, 621–625 (2005).

44. 44.

Giannozzi, P. et al. QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. J. Phys. Condens. Matter 21, 395502 (2009).

45. 45.

Giannozzi, P. et al. Advanced capabilities for materials modelling with QUANTUM ESPRESSO. J. Phys. Condens. Matter 29, 465901 (2017).

46. 46.

Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865–3868 (1996).

47. 47.

Grimme, S., Antony, J., Ehrlich, S. & Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 132, 154104 (2010).

48. 48.

Tersoff, J. & Hamann, D. R. Theory of the scanning tunneling microscope. Phys. Rev. B 31, 805–813 (1985).

49. 49.

Singh-Miller, N. E. & Marzari, N. Surface energies, work functions and surface relaxations of low-index metallic surfaces from first principles. Phys. Rev. B 80, 235407 (2009).

50. 50.

Pehlke, E. & Scheffler, M. Evidence for site-sensitive screening of core holes at the Si and Ge(001) surface. Phys. Rev. Lett. 71, 2338–2341 (1993).

51. 51.

García-Gil, S., García, A. & Ordejón, P. Calculation of core level shifts within DFT using pseudopotentials and localized basis sets. Eur. Phys. J. B 85, 239 (2012).

52. 52.

Aldahhak, H. et al. X-ray spectroscopy of thin film free-base corroles: a combined theoretical and experimental characterization. J. Phys. Chem. C 121, 2192–2200 (2017).

53. 53.

Tebi, S. et al. On-surface site-selective cyclization of corrole radicals. ACS Nano 11, 3383–3391 (2017).

54. 54.

Aldahhak, H. et al. Identifying on-surface site-selective chemical conversions by theory-aided NEXAFS spectroscopy: the case of free-base corroles on Ag(111). Chem. Eur. J. 24, 6787–6797 (2018).

55. 55.

Paszkiewicz, M. et al. Unraveling the oxidation and spin state of Mn–corrole through X-ray spectroscopy and quantum chemical analysis. J. Phys. Chem. Lett. 9, 6412–6420 (2018).

56. 56.

Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 50, 17953–17979 (1994).

57. 57.

Taillefumier, M., Cabaret, D., Flank, A.-M. & Mauri, F. X-ray absorption near-edge structure calculations with the pseudopotentials: Application to the K edge in diamond and α-quartz. Phys. Rev. B 66, 195107 (2002).

58. 58.

Van Ausdall, B. R., Glass, J. L., Wiggins, K. M., Aarif, A. M. & Louie, J. A systematic investigation of factors influencing the decarboxylation of imidazolium carboxylates. J. Org. Chem. 74, 7935–7942 (2009).

## 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

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.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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.

## Rights and permissions

Reprints and Permissions

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