Surface Passivation and Positive Band-Edge Shift of P-Si(111)
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Subscriber access provided by Caltech Library C: Surfaces, Interfaces, Porous Materials, and Catalysis Surface Passivation and Positive Band-Edge Shift of p-Si(111) Surfaces Functionalized with Mixed Methyl/Trifluoromethylphenylacetylene Overlayers Miguel Cabán-Acevedo, Kimberly M. Papadantonakis, Bruce S. Brunschwig, and Nathan S. Lewis J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.0c02017 • Publication Date (Web): 30 Jun 2020 Downloaded from pubs.acs.org on June 30, 2020 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. 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ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts. is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Page 1 of 15 The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 Surface Passivation and Positive Band-Edge Shift of p-Si(111) 9 10 Surfaces Functionalized with Mixed Methyl/ 11 12 Trifluoromethylphenylacetylene Overlayers 13 † ‡ ,†,‡ 14 Miguel Cabán-Acevedo, Kimberly M. Papadantonakis, Bruce S. Brunschwig, Nathan S. Lewis 15 †Division of Chemistry and Chemical Engineering, and ‡Beckman Institute, California Institute of Technology, Pasadena, 16 California 91125, United States 17 18 19 20 ABSTRACT. Chemical functionalization of semiconductor surfaces can provide high-efficiency photoelectrochemical devices 21 through molecular-level control of the energetics, surface dipole, surface electronic defects, and chemical reactivity at 22 semiconductor/electrolyte junctions. We describe the covalent functionalization by nucleophilic addition chemistry of p-Si(111) 23 surfaces to produce mixed overlayers of trifluoromethylphenylacetylene (TFMPA) and methyl moieties. Functionalization of Cl- 24 terminated Si(111) surfaces with TFMPA moieties introduced a positive surface molecular dipole that in contact with CH3CN or Hg 25 produced a positive band-edge shift of the semiconductor relative to junctions with CH3-Si(111) surfaces. Methylation of the 26 Cl/TFMPA surfaces using methylmagnesium chloride resulted in the degradation of the TFMPA moieties, whereas methylation using 27 methylzinc chloride allowed controlled production of mixed TFMPA/methyl-terminated surfaces and permitted reversal of the order of the functionalization steps so that nucleophilic addition of TFMPA could be accomplished after methylation of Cl-Si(111) surfaces. 28 Mixed TFMPA/methyl functionalization resulted in a Si(111) surface with surface recombination velocities of 2 × 102 cm s-1 that 29 exhibited an ~ 150 mV positive band-edge shift relative to CH3-Si(111) surfaces. 30 31 32 33 I. INTRODUCTION 34 Chemical functionalization of semiconductor surfaces is a powerful route to high-efficiency photoelectrochemical devices through 35 simultaneous molecular-level control of the energetics, surface electronic defects, and chemical reactivity of semiconductor/liquid 36 junctions. Introduction of a dipole on the surface of a covalent semiconductor, such as Si, through functionalization of the surface by 37 a thin organic monolayer can shift the energy-band positions of the semiconductor relative to the contacting phase (Figure 1).1-5 Band- 38 alignment requirements and sensitivity to electronic defects at the semiconductor surface place substantial limitations on the use and 6 39 efficiency of semiconductor/electrolyte junctions. 40 Theoretical and experimental studies on Si suggest that the direction and magnitude of the surface dipole moment is dependent on 1 41 the molecular dipole properties of the attached group. Covalent surface functionalization has been demonstrated for an extensive 7-16 42 variety of semiconducting inorganic 3D substrates such as Si, Ge, diamond, GaAs, TiO2, and GaInP2. For a photocathode in contact with a redox couple in solution, the potential of the conduction-band edge (Ecb) of the semiconductor must be more negative than the 43 electrochemical potential of the contacting phase prior to equilibration; however, for high-efficiency photoconversion the difference 44 17 between the Fermi-level potential of the semiconductor (EF) and that of the contacting phase needs to be maximized. 45 Halogenation of H-Si(111) surfaces followed by alkylation with methylmagnesium chloride18 results in a fully methyl-terminated 46 (CH -Si(111)) surface that has a low surface recombination velocity (~ 20 cm s-1) and extended stability (> 500 h) in air.19-22 Compared 47 3 to hydrogen-terminated (H-Si(111)) surfaces, CH3-Si(111) surfaces are chemically robust and can be interfaced with metals without 48 the formation of deleterious metal silicides.23-25 A negative surface dipole shift of up to ~ -0.8 V has been observed for junctions 23 49 between Hg and CH3-Si(111) relative to H-Si(111). A negative surface dipole shifts the conduction and valence bands to more 50 negative potentials (electron energies closer to the vacuum level) relative to the chemical potential of a contacting phase. This shift 51 increases the built-in voltage, Vbi, and improves the band alignment of n-type silicon devices (Figure 1a-c). In contrast, a negative - 52 dipole shift reduces the band bending and built-in voltage (Vbi = EF-E(A/A )) for contacts to p-type silicon, and a positive surface 53 dipole is therefore required to position the semiconductor bands to more positive potentials relative to the contacting phase and 54 therefore produce improved photovoltages for p-type photocathodes (Figure 1d-f). 55 56 57 58 59 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 2 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Figure 1. Diagram illustrating the shift in the relative positions of the bands for (a, b, c) n-type and (d, e, f) p-type semiconductor, and the 24 contacting electrolyte layer induced by (a, d) no, (b, e) negative, and (c, f) positive dipole layers. In this figure, EC, EV, EF, and EVac represent the potentials of the conduction-band-edge minimum, valence-band-edge maximum, Fermi-level and the vacuum level, respectively. The 25 shift in the vacuum level caused by the electric field within the dipole layer is represented as . The barrier height, b, limits the attainable 26 photovoltage. 27 28 Chemical modification with mixed organic moieties is a promising strategy to functionalize inorganic semiconducting surfaces 29 when chemical passivation and target functionality cannot be achieved by a single moiety, or when multiple chemical functionalities are needed.5, 12, 20, 26-28 In the case of Si(111), favorable band-edge positions have been obtained by functionalizing p-Si with 3,4,5- 30 trifluorophenylacetylene (TFPA), an organic moiety with a positive molecular dipole.11, 29 Phenyl groups are sterically prevented from 31 binding to every Si atom on the Si(111) surface,30 so the p-Si(111) samples were sequentially functionalized with TFPA, to induce 32 the desired dipole, and then with methyl groups, to passivate surface sites not bound to TFPA. Mixing TFPA and methyl-terminated 33 sites resulted in a net positive band-edge shift relative to CH3-Si(111) surfaces and encouragingly reversed the sign of the dipole 34 relative to CH3-Si(111) surfaces. However, for photoelectrochemical junctions in acetonitrile, the mixed TFPA/methyl-functionalized 35 p-Si surfaces displayed low energy-conversion efficiencies and yielded photovoltages that were not commensurate with the measured 36 changes in barrier height. The suboptimal photoelectrochemical properties of mixed TFPA/methyl-functionalized surfaces have been 37 attributed to a relatively high surface recombination velocity (1900 cm s-1 for optimal TFPA coverage), implying that for high- 38 efficiency junctions defect passivation is required in addition to a favorable energy-band alignment. 39 We describe herein a synthetic pathway to p-Si(111) surfaces functionalized with a mixed overlayer containing 4- 40 trifluoromethylphenylacetylene (TFMPA) and methyl groups. The resulting surfaces exhibit a tunable, positive surface dipole 41 beneficial for use of p-Si(111) photocathodes, and additionally yields surfaces that have high degrees of electrical passivation. Fluoro 42 (–F)