Effect of Surface Coverage and Composition on the Stability and

May 10, 2017 - The interfacial dipole calculations show that the shift in dipole relative to the H-terminated Si(111) surface increases as a function ...
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Effect of Surface Coverage and Composition on the Stability and Interfacial Dipole of Functionalized Silicon Kara Kearney,†,⊥ Ashwathi Iyer,‡ Angus Rockett,§,⊥ Aleksandar Staykov,⊥ and Elif Ertekin*,∥,⊥ †

Department of Materials Science and Engineering, University of Illinois, 1304 West Green Street, Urbana, Illinois 61801, United States ‡ Department of Physics, University of Illinois, 1110 West Green Street, Urbana, Illinois 61801, United States § Department of Metallurgy and Materials Engineering, Colorado School of Mines, 201 Hill Hall, 1500 Illinois Street, Golden, Colorado 80401, United States ∥ Department of Mechanical Science and Engineering, University of Illinois, 1204 West Green Street, Urbana, Illinois 61801, United States ⊥ International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: A method for predicting the stability and interfacial dipole of mixed functionalized surfaces using firstprinciples density functional theory calculations is described, and calculated trends are consistent with previously published experimental data. Predicting the interfacial dipole is critical for photovoltaic and photoelectrochemical applications because the dipole can be tailored to enhance device performance by improving charge separation at the interface. To demonstrate the approach, the enthalpy of reaction and interfacial dipole as a function of coverage of 3,4,5-trifluorophenylacetylenyl (TFPA) moieties on Si(111) was analyzed for both mixed methyl/TFPA and mixed chlorine/TFPA-terminated surfaces. The enthalpy of reaction calculations show that the affinity for functionalization improves as a function of TFPA coverage for the mixed chlorine surface but instead remains constant for the mixed methyl surface across all coverages. The results indicate that the trend in enthalpy of reaction is a good predictor of the affinity for functionalization and the stability of the resulting surface. The interfacial dipole calculations show that the shift in dipole relative to the H-terminated Si(111) surface increases as a function of TFPA coverage with the mixed chlorine surface having a more positive shift than the mixed methyl across all coverages. We find that there are significant interactions between TFPA and neighboring −Cl or −CH3 moieties that increase the magnitude of the interfacial dipole. This suggests that the magnitude of an interfacial dipole can be tuned by adjusting the chemical makeup of a mixed monolayer. All trends presented in this work were validated against experimental observations found in literature for both mixed methyl/TFPA and chlorine/TFPA surfaces.

1. INTRODUCTION In theory, designing a system capable of harvesting ∼20 TW of the solar power incident on Earth would meet total global energy consumption.1 Two promising devices to achieve this are photovoltaic and photoelectrochemical (PEC) cells, which both rely on the use of a semiconductor to capture and convert incident light into free charge carriers that are separated and used to generate an electric current.2 Because device performance is extremely sensitive to the quality of the chargeseparation interface, research has focused on controlling interfacial properties through surface modification.3,4 One versatile technique is engineering the band-edge positions by controlling the direction and magnitude of the interfacial dipole through chemical attachment of molecular species to the semiconductor surface.5 © XXXX American Chemical Society

Silicon is commonly used in both photovoltaic and photoelectrochemical devices due to its low material cost and high earth abundance. Si(111) is an ideal choice for band-edge engineering due to the well-defined chemical and structural properties of the H-terminated surface.6 Despite having superior electrical properties, Si(111)−H surfaces gradually oxidize in air, which is impractical for device applications.7 However, Lewis and co-workers have shown that a 100% CH3terminated Si(111) surface resistant to oxidation can be synthesized from the Si(111)−H surface using a two-step chlorination/alkylation reaction.7,8 Lewis and co-workers have Received: January 25, 2017 Revised: May 5, 2017 Published: May 10, 2017 A

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much TFPA as possible, in terms of both interfacial dipole and resistance to oxidation. Our results further suggest, however, that steric effects during the reaction sequence make high coverages of TFPA on the CH3/TFPA surface difficult to achieve. We find that there are significant interactions between neighboring organic moieties in mixed monolayers on Si(111) that influence the magnitude of the interfacial dipole. This suggests that the effective band-edge energies of a Si surface can be engineered to have a specified value by tuning the interfacial dipole of the surface with the chemical makeup of the mixed monolayer. Additionally, we find that calculating the enthalpy of reaction associated with surface functionalization using the DFT-computed total energies is a good indicator of the degree of surface passivation. Previous computational studies have been performed to understand the electronic properties of functionalized Si(111) surfaces, which include density functional theory15−17 and numerical device modeling.18 Additionally, Lewis and Galli et al. undertook a combined theoretical and experimental study of the surface potential shift as a function of the functional group and coverage, without a mixed moiety layer, using both density functional theory (DFT) and many-body perturbation theory (MBPT).19 It was found that MBPT is far superior to DFT at calculating the absolute value of the surface dipole; however, DFT was found to be sufficient for calculating the trends in surface dipole relative to the Si(111)−H surface as a standard for various adsorbate types and coverages. In their work the surface sites not functionalized were terminated with hydrogen bonds. Our work suggests that the moiety in which the nonfunctionalized sites are terminated (−H, −CH3, −Cl, etc.) is important to consider due to interactions that may occur between neighboring functional groups.

further extended this chlorination/alkylation process to mixed methyl/alkyl monolayers.9−12 For example, mixed methyl and CH2CHCH2−Si(111) surfaces have been synthesized and shown to display similar passivation properties as the CH3− Si(111) surface.9 This suggests that as long as 100% Si−C atop bonding is achieved, the surface will resist oxidation. Despite the superior passivation properties of the Si(111)− CH3 surface, CH3-termination produces a −0.5 eV shift in the surface dipole relative to Si(111)−H, which decreases the open-circuit voltage for p-Si devices.12−14 Therefore, to improve the performance of Si(111)−CH3 for p-Si devices, a mixed monolayer that maintains 100% Si−C atop bonding while establishing a dipole more positive than Si(111)−CH3 is needed. Plymale et al. have shown that this can be achieved with a mixed methyl and 3,4,5-trifluorophenylacetylene (TFPA) surface. They have shown that varying coverages of methyl and TFPA functional groups can be obtained by using a controlled three-step halogenation/alkylation process with the following reaction sequence: Si(111)-H → Si(111)-Cl → mixed Si(111)-Cl/TFPA → mixed Si(111)−CH3/TFPA (refer to Scheme 1).12 Scheme 1. Reaction Sequences Used To Obtain Si−Cl/ TFPA and Si−CH3/TFPA Functionalizationa

2. METHODS 2.1. Input Parameters. The electronic structure of functionalized Si(111) was studied using density functional theory (DFT)20,21 with the plane-wave code VASP22−26 along with the Perdew−Burke−Ernzerhof27 exchange-correlation potential and PAW pseudopotentials.28 A 2 × n × 8 symmetrically terminated Si(111) supercell was used with a 12 Å thick vacuum layer. The size of the unit cell in the “n” direction was modulated between 4 and 14 surface lattice sites depending on the desired surface coverage. Two different types of Si(111) surfaces were simulated in this worka mixed chlorine/TFPA composition referred to as Si−Cl/TFPA and a mixed methyl/TFPA composition referred to as Si−CH3/ TFPA. The surface coverage is defined as the fractional percent of the top monolayer (ML) of silicon occupied by TFPA where 1 ML of coverage corresponds to one TFPA per every available atop Si atom. For example, the unit cells at 25% TFPA coverage (θTFPA = 0.25 ML) for both Si−Cl/TFPA and Si−CH3/TFPA are shown in Figure 1a and Figure 1b, respectively, where the size of the supercell in the n direction of the 2 × n × 8 unit cell is 4. A plane-wave cutoff of 480 eV, a Gaussian smearing of 0.2 eV, and a k-space sampling of 8 × 2 × 1 (n = 4) and 8 × 1 × 1 (n > 4) were used. All structures were relaxed using a convergence criterion of 0.02 eV/Å for forces on each atom and 10−7 eV for the energy difference between subsequent steps. Note that the TFPA rings were free to rotate to their lowest energy orientation and were not constrained to lie in a plane as suggested by Figure 1. To avoid formation of a net dipole from one surface of the simulation slab to the other, both surfaces of the slab were terminated identically.

a Si−Cl/TFPA is synthesized by alkylating Si−Cl with TFPA−Li. Si− CH3/TFPA is synthesized by alkylating Si−Cl with TFPA−Li followed by subsequent methylation with CH3−Mg−Cl.

Plymale et al. experimentally observed trends in the interfacial dipole of both mixed Si(111)−Cl/TFPA and mixed Si(111)−CH3/TFPA surfaces via electrochemical methods. For both Si(111)−Cl/TFPA and Si(111)−CH3/ TFPA, the dipole is more positive relative to Si(111)−CH3 and increases with TFPA coverage. 12 Although the dipole established for Si−Cl/TFPA was more positive than Si− CH3/TFPA across all coverages, the lack of 100% Si−C atop bonding resulted in significant oxidation and/or surface state formation.12 Therefore, the ideal surface in principle should be CH3/TFPA with the highest achievable coverage of TFPA. However, Plymale et al. have observed degradation of CH3/ TFPA surfaces with greater than 20% TFPA coverage, which they attributed to the presence of residual chlorine left behind during the reaction sequence. In this work, we show how first-principles DFT calculations can be used to predict the stability and shift in interfacial dipole of functionalized surfaces relative to the Si(111)−H surface as a function of mixed monolayer composition. As an illustration of the technique, we study the effect of composition for both mixed Cl/TFPA and CH3/TFPA surfaces and compare our results to the experimental trends discussed above. Our results confirm that the ideal functionalization is CH3/TFPA with as B

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Figure 1. Relaxed units cells at θTFPA = 0.25 surface coverage of TFPA for (a) side view of Si−Cl/TFPA and (b) side view of S−CH3/TFPA. The dark blue, brown, gray, green, and light blue atoms represent silicon, carbon, hydrogen, chlorine, and fluorine, respectively. The surface is a [111] plane.

2.2. Enthalpy of Reaction. Experimentally, Si−Cl/TFPA and Si−CH3/TFPA surfaces are synthesized by reacting alkyl lithium and alkyl gringard reagents with a Cl-terminated Si(111) surface.8,12 As shown in Scheme 1, the Si−Cl/TFPA surface is achieved by reacting Si(111)−Cl with TFPA−Li while the Si−CH3/TFPA is achieved by reacting the Si(111)− Q surface =

Cl surface with TFPA−Li followed by subsequent reaction with CH3−Mg−Cl to replace the remaining −Cl moieties with −CH3 groups. The enthalpy of reaction (Qsurface) per unit surface area for a given functionalized surface can be calculated by accounting for the energy changes associated with the ion exchange reactions indicated in Scheme 1

Eslab + 2n TFPA ECl−Li + 2nCH3ECl−Mg−Cl − (ESi−Cl + 2n TFPA E Li−TFPA + 2nCH3ECH3MgCl) 2A surface

where Eslab is the DFT-computed total energy of the slab under consideration while ESi−Cl is the DFT-computed total energy of Si−Cl. The Si−Cl surface serves as the reference for calculating the enthalpy of reaction of both Si−Cl/TFPA and Si−CH3/ TFPA. nTFPA and nCH3 are the number of −TFPA and −CH3 moieties, respectively, on the surface. ECl−Li, ETFPA−Li, ECH3−Mg−Cl, and ECl−Mg−Cl are the DFT-computed total energies of Cl−Li, TFPA−Li, CH3−Mg−Cl, and Cl−Mg−Cl, respectively. Asurface is the surface area of the slab used in the calculation of both Eslab and ESi−Cl. In eq 1, the total energies of the ionic compounds and the total surface area are multiplied by 2 because the total energy is calculated using a symmetrically terminated slab with two exposed surfaces. 2.3. Surface Dipole. Surface functionalization may result in the establishment of an interfacial dipole due to an asymmetric distribution of charge between the substrate and the attached molecules. The surface dipole was calculated here using a procedure called “nanosmoothing” in which the microscopic oscillations in the electrostatic potential are smoothed out.29 This procedure separates the surface effects from the periodic, atomic-scale oscillations in the electrostatic potential occurring in the bulk and provides a numerical value of the surface dipole, reported here in units of Debye/site, where site refers to the number of surface lattice sites in the unit cell. In this work, a

(1)

positive surface dipole is defined as a dipole moment in which the organic moiety has a partial negative charge relative to bulk silicon (Siδ+−Rδ‑). A negative surface dipole is defined as a dipole moment in which the organic moiety has a partial positive charge relative to bulk silicon (Siδ−−Rδ+). 2.4. Charge and Electron Distributions. The charge rearrangement that takes place at the silicon surface due to functionalization can be visualized qualitatively by plotting a charge density difference isosurface. The charge density difference, Δρ, is defined by eq 2 Δρ = ρtotal − (ρSi + ρmoieties )

(2)

where ρtotal is DFT-computed electronic charge density of the fully relaxed surface under consideration. ρSi is the DFTcomputed electronic charge density of a hypothetical state of the isolated Si in vacuum (moieties removed) frozen in the geometry from the relaxed total surface. ρmoieties is the DFTcomputed electronic charge density of the isolated moieties in vacuum (Si atoms removed) frozen in the geometry from the relaxed total surface. To obtain a quantitative understanding of the charge rearrangement, Bader population analysis of the charge and electron distribution was performed.30 C

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3. RESULTS 3.1. Enthalpy of Reaction and Surface Stability. To analyze the thermodynamic stability of the functionalized surfaces, the enthalpy of reaction as a function of fractional surface coverage of TFPA, θTFPA, for Si−CH3/TFPA and Si− Cl/TFPA was calculated. The results are shown in Figure 2,

Figure 3. Density of states as a function of band energy for Si−CH3/ TFPA at θTFPA = 0.25, Si(111)−CH3, and Si(111)−Cl. Although Si− CH3/TFPA exhibits an increased density of states approximately 2 eV below the valence band maximum, none of the functionalized surfaces exhibit midgap states, suggesting that functionalization does not directly contribute to surface recombination. Figure 2. Enthalpy of reaction in eV per nm2 as a function of the fractional surface coverage of TFPA molecules (θTFPA) for both Si− Cl/TFPA (red hollow squares) and Si−CH3/TFPA surfaces (black solid squares). For Si−Cl/TFPA the enthalpy of reaction decreases linearly as a function of TFPA coverage while for Si−CH3/TFPA the enthalpy of reaction remains constant. A negative enthalpy of reaction denotes stability with respect to the reference surface which is the fully functionalized Si(111)−Cl surface.

surface types, Si−Cl, Si−CH3, and Si−CH3/TFPA, we argue that the midgap recombination active sites originating from the organic moieties are not responsible for the experimentally observed trends in SRV versus θTFPA for these surfaces. With this said, we note that recombination may be increased in other ways than through states in the energy gap. For example, an increase in the density of states near the band edge could increase recombination. Compared to Si−CH3 and Si−Cl, Si− CH3/TFPA has an increased density of states concentrated approximately 2 eV below the valence band maximum. In this case, the increased density of states lies well below the band edge, which indicates this is also not the mechanism for increased recombination. Instead, the results of Figures 2 and 3 are consistent with the experimental observation that residual chlorine susceptible to the formation of surface states is responsible for the observed trends in SRV. We note from Figure 2 that as the fractional coverage of chlorine of Si−Cl/TFPA increases (lower θTFPA) the enthalpy of reaction becomes more positive. This suggests that the presence of chlorine makes the surface less stable and potentially more susceptible to the formation of surface states. The enthalpy of reaction for Si−Cl/TFPA is more positive than Si−CH3/TFPA across all coverages, indicating that the reaction sequence in which Si−Cl is replaced with Si−CH3 at fixed θTFPA is always thermodynamically favorable. This is consistent with Plymale et al.’s suggestion that steric hindrance, rather than unfavorable reaction thermodynamics, is the cause of residual chlorine observed in experiment at high θTFPA. We conclude from these results that the enthalpy of reaction is a good predictor for the stability of functionalized Si(111) as long as steric hindrance does not impede functionalization. 3.2. Surface Dipole. A positive surface dipole is defined as a dipole moment in which the silicon has a partial positive charge while the organic moiety has a partial negative charge. In principle, a fully terminated Si−Cl surface will have a more positive dipole than the fully terminated Si−CH3 surface due to the electronegativity difference between the two moieties. Adding TFPA to the surface induces an even more positive dipole moment because TFPA has a large, positive molecular dipole moment of 2.41 D due to the three C−F bonds at the

where a negative enthalpy of reaction denotes stability with respect to a fully Cl-terminated surface. For Si−Cl/TFPA, the enthalpy of reaction decreases linearly from about 0 to −4 eV/ nm2 as θTFPA increases up to 0.5 ML, indicating that the surface becomes more stable as Si−Cl bonds are replaced by Si−TFPA. On the other hand, Si−CH3/TFPA remains at a constant ∼−8 eV/nm2 as θTFPA increases, which indicates that the enthalpy of reaction is not influenced by whether the Si−C bond comes from −CH3 or −TFPA. To understand these results, we note that Plymale et al. observed experimentally that the surface recombination velocity (SRV) for Si−CH3/TFPA increases with θTFPA.12 This may indicate that TFPA renders the surface less stable and possibly introduces midgap surface states that are recombination active. However, they also observed using XPS that residual −Cl and small amounts of SiOx were found on Si−CH3/TFPA surfaces with θTFPA > 0.2 ML and θTFPA > 0.15 ML, respectively.12 They suggested that the trend in surface recombination velocity may not be coming from decreased stability and/or midgap states due to TFPA, but instead from unreacted Si−Cl groups left on the surface, which are known to be susceptible to the formation of surface states.12,9−11 They attributed the presence of residual Cl on the surface to steric hindrance that limits the passivation of Si−Cl sites with −CH3. The results in Figure 2 indicate that the observed increase in SRV with θTFPA for Si−CH3/TFPA is not due to a decrease in the surface stability with TFPA, which is found to remain constant across all compositions. To rule out that the high SRV may be originating from surface states coming from TFPA, the density of states of the functionalized surfaces was calculated. Figure 3 shows the density of states for Si−Cl, Si−CH3, and Si−CH3/TFPA at θTFPA = 0.25 ML. As there is a lack of surface states for all D

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Figure 4. (a) Surface dipole shifts relative to Si(111)−H as a function of the fractional surface coverage of TFPA (θTFPA) for both Si−Cl/TFPA (red hollow squares) and Si−CH3/TFPA (black solid squares). Values are reported as Debye/site where “site” is the number of surface lattice sites in the unit cell configuration. The dipole shifts for 100% Cl- and 100% CH3-terminated surfaces are plotted at θTFPA = 0 for Si−Cl/TFPA and Si−CH3/ TFPA, respectively. The dipole shift for a 100% TFPA-terminated surface is plotted at θTFPA = 1. For both surfaces the dipole shift becomes more positive as θTFPA increases, which indicates a stronger induced positive dipole on the surface. The red dashed and black solid lines represents the expected total surface dipole shift if depolarization effects are considered while lateral interactions are neglected. (b) The magnitude of the surface dipole shift due to interactions (pinteraction) between −Cl/TFPA and − CH3/TFPA as represented by the red hollow squares and black solid squares, respectively. pinteraction was quantified as the deviation of the total surface dipole shifts from the expected surface dipole shifts that neglect lateral interactions as shown in panel (a).

Figure 5. Comparison of (a, d) the charge density difference isosurfaces, (b, e) Bader analysis, and (c, f) electronic charge redistribution upon interactions between neighboring moieties for Si−Cl/TFPA and Si−CH3/TFPA at θTFPA = 0.25, respectively. The red and yellow isosurfaces are for charge density differences greater or less than +0.001 and −0.001 e, respectively. A positive isosurface difference is a loss of electrons while a negative isosurface is a gain of electrons. For the Bader analysis, a positive charge indicates a loss of electrons while a negative charge indicates a gain of electrons.

= 0.0 ML, are +0.37 and −0.29 D/site for Si−Cl/TFPA and Si−CH3/TFPA, respectively, which are within 0.01 eV of previous theoretical calculations by Galli et al.19 The surface dipole shift for a 100% TFPA-terminated surface is plotted at θTFPA = 1.0 ML. For both surface types the surface dipole shift becomes more positive than Si−Cl or Si−CH3, respectively, as θTFPA increases, which indicates a stronger induced negative charge on the organic moieties and positive charge in the underlying Si. The surface dipole is more positive for Si−Cl/

top of the phenyl ring oriented with a significant portion of the dipole moment normal to the surface. The surface dipole shifts relative to 100% H-terminated Si(111) as a function of θTFPA for Si−Cl/TFPA and Si−CH3/ TFPA are shown in Figure 4a. The magnitude of the shifts are reported in units of Debye/site and are normalized by the number of surface lattice sites in the unit cell used during the calculation. The calculated surface dipole shifts for 100% chlorine and 100% methyl-terminated surfaces, plotted at θTFPA E

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are closest to the TFPA molecule donate charge to the triple carbon bond, which redistributes charge in a way that aligns in the same direction as the total positive dipole. In the case of Si−Cl/TFPA (Figure 5c), the hydrogens donate charge to the chlorines followed by a redistribution of charge from carbon-4 to carbon-3,5 as indicated by the +0.33 charge on carbon-4 in Figure 5b. This charge redistribution creates a dipole between carbon-4 and carbon-7 that aligns in the opposite direction as the total positive dipole, which explains the smaller induced dipole due to interactions observed on the Si−Cl/TFPA surface relative to Si−CH3/TFPA.

TFPA than Si−CH3/TFPA surfaces for all coverages. Plymale et al. observed this same increase in dipole as a function of θTFPA from J−V and Cdiff−V data for n- and p-Si−CH3/TFPA/ Hg junctions.12 In addition, Plymale et al. observed, using photoelectrochemical measurements in CH3CN, that n-Si−Cl/ TFPA samples exhibited a greater shift in the open-circuit potential (Eoc) than n-Si−CH3/TFPA samples.12 They attributed this behavior to residual −Cl sites on n-Si−Cl/ TFPA contributing to the positive shift in the band-edge position,12 which is in agreement with the trends present in our DFT calculations. Expected dipole shifts that incorporate depolarization effects but neglect lateral interactions between TFPA and CH3/Cl are included in Figure 4a between the data points at θTFPA = 0.0 ML and θTFPA = 1.0 ML for the Si−Cl/TFPA (red dashed line) and Si−CH3/TFPA (black solid line) surfaces. These shifts were quantified by calculating the surface dipole of only TFPA or CH3/Cl as a function of coverage of the single moiety on Si(111) as shown in Figure S1. Then, the expected total dipole in the absence of interactions between different moieties was obtained by summing the isolated contribution of each species. The actual dipole shifts calculated for the mixed monolayers (red hollow squares and black solid squares) deviate from the expected dipole shifts that neglect lateral interactions, and the magnitude of deviation (pinteraction) is shown in Figure 4b. The amount of dipole due to lateral interactions peaks at about θTFPA = 0.25−0.30 ML for both surfaces and the interactions between −TFPA and −CH3 are larger than the interactions between −TFPA and −Cl across all coverages. These results suggest that when functionalizing surfaces with mixed monolayers, it is important to consider the interaction between neighboring moieties when engineering a particular magnitude of surface dipole. To visualize where the interactions are occurring between −CH3/Cl and −TFPA, the charge density difference isosurfaces for Si−CH3/TFPA and Si−Cl/TFPA at θTFPA = 0.25 ML were plotted. As shown in Figure 5a and 5d (left panels), the interactions are occurring via hydrogen bonding and hydrogen−π interactions for Si−Cl/TFPA and Si−CH3/TFPA, respectively. On the Si−Cl/TFPA surface the bonding occurs between the chlorines and the hydrogens on the 3,5 positions of aromatic ring. On the Si−CH3/TFPA surface the hydrogen−π interactions occur between the hydrogens on the methyl group and the triple carbon bond on TFPA. To quantify the amount of charge extracted from Si−Cl/ TFPA and Si−CH3/surfaces, Bader population analysis was performed. The results are shown in Figure 5b and 5e (middle panels) for Si−Cl/TFPA and Si−CH3/TFPA, respectively, where a positive charge indicates a loss of electrons while a negative charge indicates a gain of electrons. In addition to the mixed surfaces, Bader analysis was also performed for the 100% −Cl, −CH3, and −TFPA terminated surfaces. The results revealed that approximately one electron is extracted per −Cl, −CH3, or − TFPA group regardless of if the functional group is on a mixed or pure surface. Therefore, the increase in dipole observed for the mixed compositions is not due to an increase in the total number of electrons extracted from the bulk Si but instead is due to charge rearrangement within the functional groups. The charge rearrangement obtained from Bader analysis due to interactions between −Cl/CH3 and −TFPA is shown schematically in Figure 5c and 5f (right panels). In the case of Si−CH3/TFPA, the two hydrogens on the methyl group that

4. CONCLUSIONS The findings of the present work are as follows: (1) In addition to predicting the surface dipole and barrier height of the functionalized semiconductor, DFT can be used to predict the stability of mixed moiety functionalized surfaces by considering the reaction sequence and calculating the relative enthalpy of reaction. (2) The effect of the chemistry of mixed surface functionalizations can be captured by calculating the theoretical trends in surface dipole with composition and coverage of the functionalization using DFT. Therefore, DFT represents an effective tool for engineering surface dipoles on Si(111). (3) Neighboring moieties on Si(111) surfaces have been found to be close enough to have significant interactions that influence the total magnitude of the surface dipole. Therefore, it is important to consider the chemical makeup of all surface sites when performing DFT calculations on Si(111) surfaces.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b00791.

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Dipole calculations neglecting lateral interactions (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 217-333-8175. ORCID

Kara Kearney: 0000-0003-1124-3369 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the Japanese Ministry of Education, Culture, Sports, Science and Technology. This material is also based upon work supported by the National Science Foundation under Grant No. 1545907.

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REFERENCES

(1) Smil, V. In Energy in Nature and Society: General Energetics of Complex Systems; MIT Press: Cambridge, MA, 2008; p 496. (2) Tan, M. X., Laibinis, P. E., Nguyen, S. T., Kesselman, J. M., Stanton, C. E., Lewis, N. S. Principles and Applications of Semiconductor Photoelectrochemistry. In Progress in Inorganic Chemistry; Karlin, K. D., Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 1994; Vol. 41.

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DOI: 10.1021/acs.jpcc.7b00791 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C (3) Lewis, N. Chemical Control of Charge Transfer and Recombination at Semiconductor Photoelectrode Surfaces. Inorg. Chem. 2005, 44, 6900−6911. (4) Guijarro, N.; Prévot, M. S.; Sivula, K. Surface Modification of Semiconductor Photoelectrodes. Phys. Chem. Chem. Phys. 2015, 17, 15655. (5) Smith, W. A.; Sharp, I. D.; Strandwitz, N. C.; Bisquert, J. Interfacial Band-Edge Energetics for Solar Fuels Production. Energy Environ. Sci. 2015, 8, 2851−2862. (6) Maldonado, S.; Plass, K. E.; Knapp, D.; Lewis, N. S. Electrical Properties of Junctions between Hg and Si(111) Surfaces Functionalized with Short-Chain Alkyls. J. Phys. Chem. C 2007, 111 (48), 17690−17699. (7) Wong, K. T.; Lewis, N. S. What a Difference a Bond Makes: The Structural, Chemical, and Physical Properties of Methyl-Terminated Si(111) Surfaces. Acc. Chem. Res. 2014, 47, 3037−3044. (8) Bansal, A.; Li, X.; Lauermann, I.; Lewis, N. S.; Yi, S. I.; Weinberg, W. H. Alkylation of Si Surfaces Using a Two-Step Halogenation/ Grignard Route. J. Am. Chem. Soc. 1996, 118, 7225−7226. (9) O’Leary, L. E.; Johansson, E.; Brunschwig, B. S.; Lewis, N. S. Synthesis and Characterization of Mixed Methyl/Allyl Monolayers on Si(111). J. Phys. Chem. B 2010, 114, 14298−14302. (10) O’Leary, L. E.; Strandwitz, N. C.; Roske, C. W.; Pyo, S.; Brunschwig, B. S.; Lewis, N. S. Use of Mixed CH3-/HC(O)-CH2CH2Si(111) Functionality to Control Interfacial Chemical and Electronic Properties During the Atomic-Layer Deposition of Ultrathin Oxides on Si(111). J. Phys. Chem. Lett. 2015, 6, 722−726. (11) O’Leary, L. E.; Rose, M. J.; Ding, T. X.; Johansson, E.; Brunschwig, B. S.; Lewis, N. S. Heck Coupling of Olefins to Mixed Methyl/Thienyl Monolayers on Si(111) Surfaces. J. Am. Chem. Soc. 2013, 135, 10081−10090. (12) Plymale, N. T.; Ramachandran, A. A.; Lim, A.; Brunschwig, B. S.; Lewis, N. S. Control of the Band-Edge Positions of Crystalline Si(111) by Surface Functionalization with 3,4,5-Trifluorophenylacetylenyl Moieties. J. Phys. Chem. C 2016, 120, 14157−14169. (13) Hunger, R.; Fritsche, R.; Jaeckel, B.; Jaegermann, W.; Webb, L. J.; Lewis, N. S. Chemical and Electronic Characterization of MethylTerminated Si(111) Surfaces by High-Resolution Synchrotron Photoelectron Spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 72. (14) Grimm, R. L.; Bierman, M. J.; O’Leary, L. E.; Strandwitz, N. C.; Brunschwig, B. S.; Lewis, N. S. Comparison of the Photoelectrochemical Behavior of H-Terminated and Methyl-Terminated Si(111) Surfaces in Contact with a Series of One-Electron, OuterSphere Redox Copules in CH3CN. J. Phys. Chem. C 2012, 116 (44), 23569−23576. (15) Li, Y.; Galli, G. Electronic and Spectroscopic Properties of the Hydrogen-Terminated Si(111) Surface From Ab Initio Calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 045321. (16) Aliano, A.; Li, Y.; Cicero, G.; Galli, G. Structural and Electronic Properties of the Methyl-Terminated Si(111) Surface. J. Phys. Chem. C 2010, 114 (27), 11898−11902. (17) Arefi, H. H.; Fagas, G. Chemical Trends in the Work Function of Modified Si(111) Surfaces: A DFT Study. J. Phys. Chem. C 2014, 118 (26), 14346−14354. (18) Kearney, K. L.; Rockett, A. A. Simulation of Charge Transport and Recombination across Functionalized Si (111) Photoelectrodes. J. Electrochem. Soc. 2016, 163 (7), H598−H604. (19) Li, Y.; O’Leary, L. E.; Lewis, N. S.; Galli, G. Combined Theoretical and Experimental Study of Band-Edge Control of Si through Surface Functionalization. J. Phys. Chem. C 2013, 117 (10), 5188−5194. (20) Hohenberg, P.; Kohn, W. Density Functional Theory. Phys. Rev. 1964, 136, B864−B876. (21) Kohn, W.; Sham, L. J. Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140, A1133−1138. (22) The VASP Site. https://www.vasp.at/ (accessed October 7, 2016).

(23) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47 (1), 558− 561. (24) Kresse, G.; Hafner, J. Ab Initio Molecular-dynamics Simulation of the Liquid-Metal-Amorphous-Semiconductor Transition in Germanium. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49 (20), 14251−14269. (25) Kresse, G.; Furthmüller, J. Software VASP, Vienna. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54 (11), 11169−11186. (26) Kresse, G.; Furthmüller, J. Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-wave Basis Set. Comput. Mater. Sci. 1996, 6 (1), 15−50. (27) Perdew, J. P.; Burke, K.; Ernzerhof, M. Fluid Vesicles in Shear Flow. Phys. Rev. Lett. 1996, 77, 3865−3868. (28) Blöchl, P. E. Projector Augmented-wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50 (24), 17953−17979. (29) Junquera, J.; Cohen, M. H.; Rabe, K. M. Nanoscale Smoothing and the Analysis of Interfacial Charge and Dipolar Densities. J. Phys.: Condens. Matter 2007, 19, 213203. (30) Tang, W.; Sanville, E.; Henkelman, G. A Grid-Based Bader Analysis Algorithm without Lattice Bias. J. Phys.: Condens. Matter 2009, 21, 084204.

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DOI: 10.1021/acs.jpcc.7b00791 J. Phys. Chem. C XXXX, XXX, XXX−XXX