Bandgap Tuning of Silicon Quantum Dots by Surface Functionalization

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Bandgap Tuning of Silicon Quantum Dots by Surface Functionalization with Conjugated Organic Groups Tianlei Zhou, Ryan T Anderson, Huashan Li, Jacob Bell, Yongan Yang, Brian P Gorman, Svitlana Pylypenko, Mark T. Lusk, and Alan Sellinger Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl504051x • Publication Date (Web): 14 May 2015 Downloaded from http://pubs.acs.org on May 15, 2015

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Bandgap Tuning of Silicon Quantum Dots by Surface Functionalization with Conjugated Organic Groups Tianlei Zhou,*, † Ryan T. Anderson, †, §, Huashan Li,‡ Jacob Bell,† Yongan Yang,† Brian P. Gorman, §,‡‡ Svitlana Pylypenko, †,§ Mark T. Lusk,‡ Alan Sellinger*, †, § †

Department of Chemistry and Geochemistry, ‡Department of Physics, §Materials Science

Program, ‡‡Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado 80401, United States

ACS Paragon Plus Environment

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ABSTRACT

The quantum confinement and enhanced optical properties of silicon quantum dots (SiQDs) make them attractive as an inexpensive and non-toxic material for a variety of applications such as light emitting technologies (lighting, displays, sensors) and photovoltaics. However, experimental demonstration of these properties and practical application into optoelectronic devices have been limited as SiQDs are generally passivated with covalently bound insulating alkyl chains that limit charge transport. In this work we show that strategically designed triphenylamine-based surface ligands covalently bonded to the SiQD surface using conjugated vinyl connectivity results in a 70 nm red shifted photoluminescence relative to their decylcapped control counterparts. This suggests that electron density from the SiQD is delocalized into the surface ligands to effectively create a larger hybrid QD with possible macroscopic charge transport properties.

KEYWORDS: Si quantum dots, type-II energy level aligned interface, band gap tuning, CT state, hydrosilylation


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Quantum dots (QDs) have enormous potential for a variety of applications based on their unique properties such as slow cooling of hot carriers, multiple-exciton generation, tunable absorption/emission, and low-cost solution processing possibilities.1-4 Of the many QD families studied to date, silicon (Si)QDs are one of the most promising candidates because of silicon’s relatively low cost, industrial maturity, high earth abundance, and non-toxic/environmentally friendly properties. As a result, SiQDs have significant advantages over other promising QD materials based on relatively toxic elements such as PbS(Se) and CdSe(Te).5-7 Similar to many other QD materials, the SiQD optical gap can be tuned by their size based on the quantum confinement effect. Examples of SiQDs with tunable emission from blue to red have been reported.7-11 Despite this, the absorption of SiQDs in the visible spectrum is very weak due to their indirect band structure.12 For example, the major absorption band of red emitting SiQDs is located at the UV/near UV wavelength region.13 This property would limit the absorption of visible wavelength photons to yield photovoltaics with low power conversion efficiencies.6,14-20 In general, SiQDs are prepared with a surface of silicon hydride groups that will be quickly oxidized to silicon hydroxyl/silicon oxide in the presence of oxygen and/or water (ambient conditions).6, 21, 22 In order to stabilize SiQDs, procedures have been developed to functionalize the surface with groups that will protect from surface oxidation and render the QDs soluble for solution processing.16, 17, 23-36 Organic alkyl chains are one of the most common groups used to functionalize SiQDs through the formation of Si-C bonds using the hydrosilylation reaction of alkenes/alkynes with the Si-H functionalized surface. To date, the influence of alkyl chains on the optical properties of SiQDs is very limited and only small changes have been reported.37-39 Ethyl termination of Si142 dots and allylamine termination of Si35 dots were predicted to result in a gap reduction of 50 meV.37, 39 In principle, alkyl chains have a minimal effect on the optical


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gap of SiQDs because of their Type-I energy level alignment and weak molecular orbital coupling to SiQDs (Fig. 1, left). In order to achieve a Type-II energy alignment with SiQDs, it is important to have a direct low-energy transition involving the highest occupied molecular orbital (HOMO) of the organic ligand with the lowest unoccupied molecular orbital (LUMO) of the SiQDs (Fig. 1, right). This was predicted by our previous theoretical study showing that a TypeII aligned organic/SiQD system exhibited lower energy and enhanced absorption resulting from direct excitation that generates a charge transfer (CT) state.40 Besides the proper Type-II energy alignment, the conjugated vinyl group that covalently links the organic ligand and the SiQD is crucial in achieving a directly generated CT state because of its ability to improve the molecular orbital overlap between the organic ligand and the SiQD.40 To date we have found very few reports that link an aromatic ligand to the SiQD using a vinyl linkage, and none combining computation and experimental results.4,

41

However, the styryl

ligand considered was not designed to form a Type-II energy alignment with SiQDs and only a slightly broadened PL was observed; both Type-II energy alignment and conjugated bridges are essential for effective tuning of SiQDs optical properties by peripheral ligands.40 Reported here is the design and synthesis of 4-ethynyl-N,N-bis(4-methoxyphenyl)aniline (MeOTPA) as the organic molecule to functionalize SiQDs. The triphenylamine moiety is a typical electron rich material that has a comparatively high-lying HOMO level compared with many other organic materials used for organic electronics devices. A para-substitution of electron donating methoxy groups can further raise the HOMO level of MeOTPA and make it high enough (–4.46 eV, 278 nm) to form the Type-II energy alignment with SiQDs. Furthermore, the hole transport properties of the triphenylamine-based ligand may facilitate hole transport in the double super-exchange system previously reported by our group.40, 42 The terminal alkyne


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functional group was added to the triphenylamine so as to form vinyl connective bridges with the Si-H sites of the SiQD upon hydrosilylation chemistry. A theoretical study of the molecular orbital energy of MeOTPA functionalized SiQDs was carried out based on the Time-Dependent Density Functional Theory (TDDFT). The computational methodology is the same as that detailed in our earlier study.40 A SiQD (Si849H344) with a diameter of 3.1 nm (Fig. 2) was chosen as a model system. In the computational study, only four MeOTPA molecules were considered because minimal changes were observed in the optoelectronic analysis of smaller dots using more than four MeOTPA attachments. Specifically, changing the number of MeOTPA ligands from 4 to 8, 12, and 16 did not significantly change the absorption peaks although it does influence the shape of the spectra at the low energy edge because of the localization of low-energy orbitals. In addition, a comparison of the optoelectronic properties of small dots with decyl and hydride passivation gave almost identical results. This implies that a direct comparison can be made between our computationally generated dots with hydrogen passivation and our experimental setting in which decyl passivation is employed. The computationally predicted influence of MeOTPA functionalization is shown in Fig. 2, where frontier orbitals are shown for hydrogen (left) versus MeOTPA (right) termination. The latter shows a significantly lower optical gap of 1292 nm (0.96 eV) as compared with those having only hydrogen termination (1033 nm, 1.20 eV). This 0.24 eV (259 nm) red-shift is consistent with our earlier prediction of a Type-II aligned organic/SiQD hybrid system.40 The results are summarized in Table 1. To experimentally verify our theoretical predictions, the MeOTPA ligand was synthesized via a Sonogashira reaction of trimethylsilylacetylene with 4-iodo-N,N-bis(4-methoxyphenyl)aniline


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followed by a potassium hydroxide removal of the trimethylsilyl group to obtain the terminal alkyne (Fig. 3a). The decyl and MeOTPA functionalized SiQDs were prepared using thermal hydrosilylation, (Fig. 3b).6 After passivation by MeOTPA or 1-decene, the resultant SiQDs became very soluble in common solvents such as dichloromethane, qualitatively indicating that surface functionalization had occurred (Fig. 3b). FT-IR was used to follow the MeOTPA reaction on the SiQD surface. In Fig. S1a, the successful preparation of MeOTPA functionalized SiQD is suggested by the formation of -SiC=C- stretching bands at 1599 cm–1, although this also overlaps with the aromatic -C=C- bonds from MeOTPA.4, 41 Additional evidence comes from the presence of aromatic C-H stretching bands around 3000 cm–1 and MeOTPA feature absorption at about 1500, 1240, 1035 and 825 cm–1. Evidence of unreacted Si-H is shown in the 2100 cm-1 region, which is not surprising as total ligand coverage of the SiQD surface is unlikely due to steric issues and is commonly seen in other SiQD hydrosilylation systems. Furthermore, no CC-H stretch at 3283 cm–1 and CC at 2100 cm–1 (Fig. S1b) are observed when compared with MeOTPA starting materials as shown in Fig. S1b. The absence of MeOTPA starting material also indicates the MeOTPA SiQDs have been well purified. Fig. S1c shows the FT-IR of decyl-passivated SiQDs. The absorption bands at 2920, 1465 and 1378 cm–1 are features from the alkyl stretching and deformation demonstrating surface alkylation of SiQDs. The absence of -C=C- stretch at 1641 cm–1 (Fig. S1d) also indicates that 1-decene has been thoroughly removed during the purification process despite also being used in large excess. As is the case for the MeOTPA SiQDs, a minor and broad peak at 2088 cm–1 results from unreacted Si-H on the SiQD surface. In the FT-IR of many reported SiQDs passivated with alkyl chains, the intense and structureless band around 1080~1090 cm–1 is commonly ascribed to Si-O-Si from surface oxidation.43, 44 A similar relatively broad peak was


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also observed in our decyl-passivated SiQDs at 1031 cm–1, that is quite different from the reported values (>50 cm–1). It has been reported that partial oxidation of SiHx surfaces could shift the hydride stretching band to higher energy with a characteristic stretch at 2250 cm–1 for O3SiH, while formation of Si-C bonds has been predicted to shift the frequency to lower energy.41 It was also reported that the vibration of Si-O not only appears at 1090 cm–1, but also at ~460 cm–1 with an intense but smaller band of 60% less in absorption intensity.43 This could be very useful and straightforward for the evaluation of the oxidation degree of SiQDs. To make sure that the 460 cm–1 band can be used as reference for our hybrid SiQDs, our hydride SiQDs were intentionally oxidized by heating them in open air for 24 hours, and the FT-IR spectrum was subsequently investigated. As seen in Figure S1e, the similar intense and broad absorption band at 1081 cm–1 and the smaller band at 458 cm–1 were observed in such oxidized SiQDs, indicating the reference absorption band around 460 cm–1 should be applicable to our SiQDs as well. Thus, the absence of an observable 460 cm–1 absorption band in the FT-IR spectra of decyl and MeOTPA passivated SiQDs indicates there was no significant oxidation in our materials. It also further confirms that the 1031 cm–1 absorption band that we observed in our SiQD materials is not from exhaustive oxidation, but rather sub-oxide formation.36 FT-IR of our starting hydride terminated SiQDs are shown in Fig. S1f showing the characteristic Si-H at 2098 cm-1. A small peak around 1030 cm-1 is observed indicating surface sub-oxides. Some C-H absorption bands are seen in the 3000 cm-1 region from residual solvent when depositing the sample on the ATR crystal. In both of our SiQD systems we are not so concerned with some oxidation on the surface as this will be inevitable in practical applications. As shown in the next sections, the surface oxidation does not seem to affect the new optoelectronic properties associated with attaching our conjugated ligands.


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Further evaluation of functionalized SiQDs was performed using X-ray photoelectron spectroscopy (XPS). The size of the functionalized SiQDs is smaller than the sampling depth of XPS, therefore XPS analysis provides bulk rather than surface composition. SiQDs functionalized with MeOTPA show ~ 72 % carbon, 14 % silicon, 11 % oxygen, and 3 % nitrogen (Table S1). XPS provides solid confirmation of surface functionalization demonstrating a carbon to nitrogen ratio of 25 that is only slightly higher than the value expected based on stoichiometry of MeOTPA of 22. Based on the stoichiometry of MeOTPA, at least 6% of oxygen should be associated with ligand, thus 5% from surface oxidation. The XPS ratio of C/Si is slightly higher than expected for a 2.8 nm SiQD with 80% coverage (65 % carbon, 19 % silicon, 7.8 % oxygen and 3.4 % nitrogen), which can be explained by the presence of adventitious carbon and sampling multiple layers of quantum dots rather than monolayer. Deconvolution of high-resolution C1s spectra shown in Fig. 4a1 provides information about the relative amount of C=C, C-N, C-O, C=O, and O-C=O, and along with the shake-up feature located at ~291 eV further confirms a match between expected and observed structures of the MeOTPA-SiQD. In comparison, C1s spectra acquired from decyl functionalized SiQDs shows C-C and various C-O species, and no shake-up feature, confirming lack of conjugation in the structure of the ligand (Fig. 4a2). Both samples show peak binding energy lower than those of C-C and C=C groups, suggesting formation of bonds between C and Si. High-resolution Si 2p spectra representative of MeOTPA-SiQDs is shown in Fig. 4b. The spectrum is fit with 4 peaks, each consisting of two components, 2p3/2 and 2p1/2, separated by 0.6 eV. The 2p3/2 component of the first doublet located at 99.6 eV is attributed to Si(0).45 The second 2p3/2 component located at 100.4 eV is due to Si-C species, further corroborating formation of bonding between Si and C atoms of the ligand.42, 46, 47 Doublet with 2p3/2 component located at 102.4 eV is due to Si-Ox species, i.e.


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suboxides.42, 48,

49

A small component at 103.0 eV could also be attributed to Si-Ox species,

which are more similar to those observed in SiO2. Based on these assignments, the amounts of Si (0), Si-C, and Si-Ox species are estimated at 58±3.3 %, 18 ±1.0 %, and 23.9±4.3 % (which normalized to the total amount of measured silicon corresponds to 7.9, 2.5 and 3.25 %, respectively). Considering the total amount of oxygen of 11% and the fact that at least 6% of oxygen is associated with ligand, the amount of oxygen that could be associated with SiQD is ~5 %. Based on the amount of oxygen bonded to Si (5%) and amount of Si bonded to oxygen (3.25%), the average ratio of oxygen atoms bonded to Si is 1.5, which is more typical of suboxides as oppose to fully oxidized Si in SiO2, The spectrum of Si 2p acquired from decyl functionalized SiQDs is fairly similar to that of MeOTPA-SiQDs with small differences related to distribution of Si-Ox bonds (Fig 4b2). This difference is not surprising considering that some heterogeneity in the Si-Ox species is observed even when comparing several analysis areas for the same sample/batch. Absorption and photoluminescence (PL) spectra of MeOTPA- and decyl-passivated SiQD dichloromethane solutions were measured and presented in Fig. 5. Photoluminescence excitation (PLE) spectra for MeOTPA ligand, MeOTPA- and decyl-passivated SiQDs are provided in Fig. S2. The MeOTPA-SiQDs exhibit an enhanced absorption in the range from 290 - 450 nm compared with the decyl-passivated dots. Some of this enhanced absorption results from the newly emerged shoulder absorption at 295 nm that is predicted to be the direct absorption transition from the MeOTPA ligand attached to the SiQD.40 In the region from 350-450 nm, the absorption intensity of MeOTPA-SiQDs has a slower decay than that of decyl-passivated dots, implying the optical gap of MeOTPA dots has been reduced. While the absorption bands of the SiQDs are relatively structureless and do not show an abrupt edge, it is clear from Fig. 5a that the


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band edge for MeOTPA-SiQD has been red-shifted by at least 75 nm over the decyl-SiQD analog. In the PL spectra (Fig. 5b), the decyl-SiQD emission peak at 679 nm (1.83 eV) suggests that the mean dot diameter is approximately 3.1 nm.50,

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This agrees well with the transmission

electron microscopy (TEM) results of MeOTPA-SiQDs as shown in Fig. 6. A size count histogram (Fig. 6a and S8) for 329 dots reveals an average size of 5.0 nm with a mode of 2.8 nm, both consistent with the 3.1 nm size suggested from the decyl-SiQD PL results (same hydride terminated SiQD precursor used for both materials). Consistent with the reduced optical gap observed in the absorption spectra, the normalized PL spectrum of MeOTPA-SiQDs (peak at 749 nm (1.66 eV)) shows a significant red shift of 70 nm versus the decyl-SiQDs. This 0.17 eV change in the emission peak is in qualitative agreement with our computational prediction of 0.24 eV for 3.1 nm SiQDs. The 72 meV difference is smaller than the uncertainty associated with the TDDFT methodology as applied to SiQDs.52 These experimental results strongly suggest that our computational predictions of the reduced optical gap in such Type-II energy level aligned hybrid SiQDs material are valid. Hydride terminated SiQDs that are significantly larger than what is studied here are expected to have HOMO levels that result in a Type-I interface between dot and MeOTPA. The direct excitation from the MeOTPA HOMO to SiQD LUMO loses dominance with the independent excitation from the SiQDs and organic ligand becoming stronger. This is consistent with the 295 nm shoulder in the absorption spectrum of MeOTPA-SiQDs, indicative of a large portion of independent excitation transitions associated only with the ligand. It should also be noted that, in contrast to the broadened and slightly red-shifted PL observed in styrene functionalized SiQDs,4 the PL of MeOTPA-SiQDs has completely shifted away from


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the PL of decyl-passivated SiQDs. This indicates the CT state dominates the excited state of MeOTPA-SiQDs. In other words, in addition to indirect excitons, direct excitons generated from the independent absorption transition of SiQDs dissociate into CT states. This is evidenced by red-shifted PL that comes primarily from direct relaxation of the CT state. We conclude from this that a Type-II aligned hybrid material system will lead to efficient exciton dissociation, one of the most important steps in solar cell photo-conversion. In order to further explore the contribution of the MeOTPA ligand on the SiQD systems, we performed cyclic voltammetry (CV) on MeOTPA-SiQD (Figure S5a), a model compound of MeOTPA attached to triethylsilane with a vinyl group (Figure S5b), the starting MeOTPA ligands (Figure S6c) and ferrocene as a control (Figure S5d).

The HOMO level for the

MeOTPA-SiQD of -4.46 eV differs substantially from the model compound (-4.66 eV) and starting ligand (-4.76 eV) suggesting that the oxidized MeOTPA ligand is being influenced by the SiQD, hence the MeOTPA and SiQD are electronically communicating through the vinyl linkage. We do not see any CV response on the decyl SiQD material. In summary, we demonstrate by using both computation and experimentation, that strategically designed aromatic amine ligands, linked by conjugation to SiQD surfaces, participate in charge transport in such hybrid systems.

This has led to an unprecedented 70 nm red shifted

photoluminescence versus their decyl terminated analogues and a 300 meV shift in the HOMO level towards vacuum from cyclic voltammetry.

We believe this result can lead to new

opportunities for SiQD application in the optoelectronic community and also evidence of using computation to help direct the design and synthesis of new materials.


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LUMO

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LUMO

HOMO HOMO QD

Ligand

Type-I

QD

Ligand Type-II

Figure 1. Electron transition in Type-I (left) and Type-II (right) molecular orbital energy level aligned organic functionalized SiQDs hybrid systems.

Hydride Passivation

MeOTPA Functionalization

Figure 2. HOMO (red) and LUMO (green) isosurfaces, from TDDFT analysis, of a 3.1 nm Si849H344 QD capped by (left) only hydride and (right) by hydride and four MeOTPA ligands. Isosurfaces are for a fixed value of the absolute value of electron orbitals, 0.009 Å-3/2.


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H 3CO

(a)

H 3CO

Pd(PPh 3)Cl2, CuI, TEA N

I

N

Si

+

Si

toluene

(2)

H 3CO

H 3CO

92% H 3CO

H 3CO KOH N

N

Si methanol/THF H 3CO

H 3CO

(3) MeOTPA 91%

(b) H

H

Si

Si

HF, ethanol

H H

H

H

H

Si

H H

H H

H

Si

Si

H

H

H H

H

H

H

H

Hydride terminated SiQD SiQDs in SiOx matrix OCH 3 H 3CO

H 3CO

N H 3CO

OCH 3

N H

H

N

H H

H H

Si

H

H

H H

H H

H 3CO

H 3CO

N OCH 3

N

N

H

H

OCH 3

H 3CO

H

H

Si

H 3CO

H 3CO

mesitylene, 160 oC

OCH 3 N

H 3CO N

OCH 3

H H

H

H

H 3CO

N H 3CO

Hydride terminated SiQD

OCH 3

N

N

OCH 3 OCH 3

OCH 3 H 3CO

MeOTPA functionalized SiQD

Figure 3. (a) The synthesis of MeOTPA ligand, (b) The preparation of hydride terminated SiQDs and MeOTPA functionalized SiQDs.


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Figure 4. High-resolution C 1s (a) and Si 2p (b) spectra from SiQDs modified with (1) MeOTPA and (2) decyl.


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Normalized Absorption

(a) 1.0

MeOTPA-Passivated Si QDs in DCM

0.8

Decyl-Passivated Si QDs in DCM MeOTPA in DCM

0.6 0.4 0.2 0.0 250

350 Wavelength (nm)

450

(b) 1.4

MeOTPA-Passivated SiQDs in DCM Decyl-Passivated SiQDs in DCM

1.2 Normalized Intensity

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1.0 0.8 0.6 0.4 0.2 0.0 600

650

700

750

800

850

900

Wavelength (nm)

Figure 5. (a) Absorption and (b) PL spectra of MeOTPA and decyl-passivated SiQDs in dichloromethane solutions.


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(a) 70

Average Size = 5.0 ± 2.1 nm Mode = 2.8 nm

60 50

Frequency

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 30 20 10 0 1

2

3

4

5

6

7

8

9

10

11

12

Size (nm)

(b)

(c)

Figure 6. TEM characterization of MeOTPA-SiQDs. (a) histogram showing the particle size distribution. (b) Bright Field TEM micrograph illustrating a representative SiQD size and


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morphology. (c) the selected area diffraction pattern of many SiQDs illustrating excellent crystallinity and primarily Si phase to be present.

Table 1. Theoretical calculation of molecular orbital energy of hydride-, decyl-passivated, MeOTPA functionalized SiQDs (3.1 nm) and MeOTPA molecule based on time dependent density functional theory (TDDFT). Energy (eV)

Si849H348

MeOTPA

-5.19

Si849H344 (MeOTPA)4 -4.89

HOMO LUMO

-4.00

-3.93

-2.10

Optical Gap

1.19 (1042 nm)

0.96 (1291 nm)

2.36 (525 nm)

-4.46


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ASSOCIATED CONTENT Supporting Information. Synthetic procedures, experimental details, FT-IR, CV, and PLE spectra, and other supporting results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: (A.S.) [email protected] *Email: (T.Z.) [email protected] ACKNOWLEDGMENT This research is supported by the Renewable Energy Materials Research Science and Engineering Center (REMRSEC) under Award Number DMR-0820518, and by startup funds (AS) from Colorado School of Mines (CSM). The authors acknowledge the Golden Energy Computing Organization at the Colorado School of Mines for the use of resources acquired with financial assistance from the National Science Foundation and the National Renewable Energy Laboratory (NREL). The authors also acknowledge the surface analysis facilities at NREL.


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For TOC Only Hydride Passivation

MeOTPA Functionalization

H3CO N H3CO

HOMO (red) and LUMO (green) isosurfaces, from TDDFT analysis of a 3.1 nm Si849H344 QD capped by (left) only hydride and (right) by hydride and 4 MeOTPA ligands.


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Bandgap Tuning of Silicon Quantum Dots by Surface Functionalization

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