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The role of surface capping molecule polarity on the optical properties of solution synthesized germanium nanocrystals Benjamin F. P. McVey, Peter O'Mara, Andrew J. McGrath, Angelique Faramus, Vineeth Benyamin Yasarapudi, Vinicius R. Gonçales, Vincent T. G. Tan, Timothy W. Schmidt, John Justin Gooding, and Richard D. Tilley Langmuir, Just Accepted Manuscript • Publication Date (Web): 29 May 2017 Downloaded from http://pubs.acs.org on May 29, 2017

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The role of surface capping molecule polarity on the optical properties of solution synthesized germanium nanocrystals B. F. P. McVeya, b, c, P. B. O'Maraa,b, A. J. McGratha,b,A. Faramusd, V. Benyamina,e, V. R. Gonçalesa, V. T. G. Tana,b, T. W. Schmidta,e, J. J. Goodinga, b, f, R. D. Tilley*a,b, c a = School of Chemistry, University of New South Wales, Sydney, NSW 2052 Australia. b = Australian Centre for Nanomedicine, University of New South Wales, Sydney NSW 2052 Australia. c = Electron Microscopy Mark Wainwright Analytical Centre, University of New South Wales, Sydney, NSW 2052 Australia. d = Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2 Canada. e = ARC Centre of Excellence in Exciton Science, University of New South Wales, Sydney, NSW 2052 Australia. f = ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, University of New South Wales, Sydney, NSW 2052 Australia. KEYWORDS: semiconductor nanocrystal, germanium, quantum dot, surface chemistry

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ABSTRACT The role surface capping molecules play in dictating the optical properties of semiconductor nanocrystals (NCs) is becoming increasingly evident. In this paper the role of surface capping molecule polarity on the optical properties of germanium NCs (Ge NCs) is explored. Capping molecules are split into two groups; non–polar and polar. The NCs are fully characterized structurally and optically to establish the link between observed optical properties and surface capping molecules. Ge NC optical properties altered by surface capping molecule polarity include emission maximum, emission lifetime, quantum yield, and Stokes shift. For Ge NCs, this work also allows rational tuning of their optical properties through changes to surface capping molecule polarity, leading to improvements in emerging Ge based bioimaging and optoelectronic devices.

INTRODUCTION Semiconductor NCs have great potential in a range of medical and optoelectronic applications from bioimaging to solar cells.1-4 To utilize semiconductor NCs in these applications often requires further modification of NC surfaces.5-7 For example, to use semiconductor NCs in bioimaging, NCs must be water-dispersible, requiring exchange of their as-synthesized hydrophobic surface capping molecules for hydrophilic molecules.5-6, 8-9 Researchers are becoming increasingly aware of the role surface capping molecules play in dictating the optical properties of semiconductor NCs.5,

10-11

Understanding how surface

capping molecules alter the optical properties is important from both a fundamental perspective and for improving optoelectronic (LEDs, solar cells) and bioimaging applications.5, 7, 9


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Proximal and distal functional groups in surface capping molecules both influence the optical properties of semiconductor NCs.11-20 Proximal groups are defined as the functional group directly bonded to the NC,5 while distal groups are defined as functional groups which terminate a surface capping molecule and interact with the solution.17,

19

How changes in

proximal or distal groups affect the optical properties of semiconductor NCs depends on the material.15-16, 21-22 For example, with CdSe NCs the proximal moiety influences the optical properties by changes in surface composition and passivation of different types of surface ions (Cd2+, Se2-).12-13, 21-24To achieve CdSe NCs with high quantum yields (QYs) and narrow emission band full width half maxima (fwhm) a balance of proximal groups that bind to both Cd2+ and Se2- ions is needed.13 For Cd2+ surface ions Lewis basic or anionic proximal groups are needed.11 For Se2- ions Lewis acidic or cationic proximal groups are needed.11 Replacing proximal bound Cd2+ ions, in the form of cadmium carboxylate complexes, with proximal amine groups decreased the QY of CdSe NCs from 10 % to 1%.13 It was discovered that proximal-bound cadmium carboxylate complexes passivate Se2- surface ions, which if left unpassivated, lead to significant losses in QY by the formation of non-radiative pathways.13 The QY could be restored by reintroducing cadmium carboxylates,13 highlighting how proximal groups control the optical properites of CdSe NCs. Group IV semiconductors Si and Ge provide low-toxicity alternatives to the commonlyencountered Cd and Pb chalcogenides.25-30 For Si NCs both distal and proximal groups of surface capping molecules can control the optical properties including absorption, QY, emission maxima and lifetime.31-35 Changing between alkyl and amine proximal groups alters the observed emission color from red to blue, increases the QY from 12 % to 32 %, and alters the emission lifetime from µs to ns.19 By altering proximal groups different types of surface states are formed Si-OX (where X = N, O, Cl) which modify electronic energy levels of the NC, causing radiative recombination to occur at the NC surface.36-38 Changing between 3 ACS Paragon Plus Environment

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alkene and carboxylic acid distal groups causes emission redshifts of 50 nm and QY losses from 5 % to 1%.17 The changes due to distal groups are less understood but hypothesized to come from the alteration of emissive states in NC core.17 Research into how surface capping molecules alter the optical properties of Ge NCs is limited compared to other semiconductor NCs such as Si.39-46 For Ge, proximal groups appear to alter the absorption and emission properties.39-40 Changing between amine and thiol proximal groups altered the band gap of Ge NCs47. The change in bandgap for Ge NCs was due to proximal thiol groups altering the electronic states.47 Red and blue emission has been reported for Ge NCs.39-40,46,48-50 Red/NIR emission in Ge NCs occurs via bandgap recombination and has only been observed with alkane proximal groups.39-40, 46 Blue emission in Ge NCs is hypothesized to occur by recombination at defect states located on NC surface.40, 51-52 Blue emission from Ge NCs has been reported in the presence of oxygen or alkane proximal groups.40, 52-53 The potential for distal groups to alter the optical properties of Ge NCs has yet to be investigated. Studying how distal groups affect the optical properties of Ge NCs is of importance for emerging Ge NC medical and optoelectronic applications.53-55 For bioimaging, Ge NCs must be made water-dispersible.53 To render NCs water-dispersible one needs to introduce polar functional groups onto surface capping molecules, while simultaneously ensuring the surface capping molecule has a short carbon backbone.9 Understanding if polar distal groups alter the emissive properties of Ge NCs, particularly, emission color, photostability, quantum yield, will affect what type of bioimaging experiments Ge NCs can be successfully utilized in.25,45 For Ge-based optoelectronic applications how non-polar distal groups alter the optical and charge transport properties of Ge NCs will determine how to construct highly efficient Ge based LEDs and


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photodetectors.54-55 Fundamentally the lack of germanium oxide (GeOx) phases in some preparations of Ge NCs suggests that Ge could have an entirely different optical response to changes in distal group compared to other distal-sensitive semiconductors such as Si.42 The purpose of this paper is to investigate the role of surface capping molecules on the optical properties of solution synthesized Ge NCs. Specifically, we focus on how the polarity of distal groups affect the absorption and emission properties of Ge NCs. Distal groups are split into two classes, non-polar and polar. Non-polar surface capping molecules are hexane and 1-octene, and the distal groups are alkyl and alkene, respectively. Polar surface capping molecules are propylamine, 2-(octylthio)-ethanoic acid and the distal groups are amine and carboxylic acid, respectively. Note that although the four different capping molecules have a differing number of carbons in carbon backbone, the change in optical properties is unlikely to be as large as for changes in distal or proximal groups.17, 19 Ge NCs were produced by reduction of halide salts, an approach which has shown good control over NC size and monodispersity.49,

53, 56-57

Using capping ligands containing both

non-polar and polar distal groups we show that the emissive properties including emission maxima, lifetime, and stokes shift of Ge NCs are altered by the polarity of the distal group. Of great interest is the surface capping induced Stokes shift, which has important implications for a wide range semiconductor NC applications including LEDs, luminescent solar concentrator devices, and bioimaging probes.6, 58-59 EXPERIMENTAL METHODS Synthesis of Ge NCs: Ge NCs were synthesized per previously developed procedures.53 Briefly, 0.914 mmol (0.5 g) of tetraoctylammonium bromide (TOAB, 98 %, Sigma Aldrich) is added to a 2-neck round bottom flask. The 2-neck flask was then attached to a Schlenk line, undergoing three cycles of evacuation and purging with nitrogen for up to 5 minutes per 5 ACS Paragon Plus Environment

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cycle. The cycles of evacuation and purging are done as the solid TOAB may adsorb atmospheric moisture. 50 mL of anhydrous toluene (99.8 %, Sigma Aldrich) was added, leaving the mixture to stir for 1 hour. The anhydrous toluene is from a “SureSeal” bottle. Before use, the bottle undergoes evacuation followed by purging with argon. 0.26 mmol (0.3 ml) of germanium (IV) tetrachloride (Sigma Aldrich) was added to the mixture and left to stir for an hour. 1.33 mmol (3.1 mL) of lithium aluminum hydride (LiAlH4, 2.0 M in THF, Sigma Aldrich) was added dropwise and left to react for 3 hours, producing hydride capped germanium nanocrystals. CAUTION GERMANE GAS CAN POTENTIALLY BE GENERATED DURING THIS STAGE. Hydride capped Ge NCs are then transferred to a degassed quartz tube via a cannula. 3 mL of degassed 1-hexene (24 mmol) (99 %, Sigma Aldrich), allylamine (40 mmol) (99 %, Acros), or 1,7-octadiene (21 mmol) (98 %, Sigma Aldrich) was then added to the mixture, and the tube was exposed to UV light (254 nm) for four hours from a custom built Rayonet UV chamber, producing alkyl, alkene, or amine terminated Ge NCs. To minimize cross-linking of octadiene, the amount of capping molecule used and the UV exposure time were optimized. Carboxylic acid capping of Ge NCs: To produce Ge NCs terminated with carboxylic acid groups, 25 mg of purified alkene terminated Ge NCs were added to 10 mL of Ndimethylformamide along with 40 mmol of 3-mercaptopropionic acid. The reaction mixture was then exposed to UV light (254 nm) for 5 hours from a custom built Rayonet UV chamber, producing carboxylic acid terminated Ge NCs. The glassware was dried overnight in an oven, and has 3 cycles of vacuum/argon purge before exposure to UV light to minimize any trace water/oxygen. Purification of Ge NCs: Unpurified Ge NCs were transferred to a round bottom flask and the solvent was removed under reduced pressure. 20 mL of hexane (HPLC grade) or MilliQ water was added to the unpurified Ge NCs and the mixture was dispersed via ultrasonication


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for 5 minutes, producing a cloudy white solution which was filtered using Millipore 0.45 µm filter paper. The filtrate was concentrated down to 2-3 mL (yellow oily appearance) via reduced pressure and run through a size exclusion column (1 cm, 60 cm) with sephadex LH20 beads (GE Life Sciences, bead size 25 – 100 µm) acting as the stationary phase and tetrahydrofuran as the solvent. The flow rate was 1 drop every 5 seconds and fractions were collected in an automated test tube collector with test tubes set to change every 50 drops. Fractions collected were checked for luminescence via the use of a handheld UV light (365 nm). Fractions that displayed luminescence were combined and concentrated under reduced pressure to yield purified Ge NCs (light yellow oil). Structural Characterization of Ge NCs: For transmission electron microscopy (TEM), a small drop of a highly concentrated (15-20 mg/mL) Ge NC solution was drop cast onto a formvar coated copper grid. The grid was left to slowly evaporate in ambient conditions. The TEM used was a Phillips CM200 operated at 200 kV. For Fourier transform infra-red spectroscopy (FTIR), a Perkin-Elmer FT-IR spectrometer was used. A KBr pellet was prepared by grinding KBr and a concentrated drop of Ge NCs. For 1H nuclear magnetic resonance (NMR) spectroscopy, a Bruker III 300 MHz spectrometer was used. Samples were prepared by dispersing Ge NCs in CDCl3 (for alkane- and alkene-terminated NCs) or D2O (for amine and carboxylic acid-terminated NCs). For X-ray photoelectron spectroscopy (XPS) a ESCALAB 220iXL spectrometer with a monochromatic Al Kα source (1486.6 eV) was used. The pressure of the operating chamber was below 10−9 mbar, and spectra were recorded in normal emission. The spot diameter was 500 µm. The resolution of the spectrometer is ca. 0.6 eV as measured from the Ag 3d5/2 signal (full width half-maximum) with 20 eV pass energy. Survey scans were carried out over 1300−0 eV range with a 1.0 eV step size, a 100 ms dwell time, and analyzer pass energy of 100 eV. High-resolution scans were run with 0.1 eV step size, dwell time of 100 ms, and analyzer pass energy set to 20 eV.


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The spectra were fitted with a convolution of Lorentzian and Gaussian profiles through Avantage software. The energies are termed as binding energies in eV and referenced to the C 1s signal (corrected to 284.8 eV). Optical Characterization of Ge NCs: For absorption measurements, hydrophobic and hydrophilic Ge NCs were dispersed in chloroform with concentrations between 1-5 mg/mL. Absorption measurements were carried out on a Cary 50 UV-Vis spectrometer. Blank measurements were carried out using chloroform. For emission measurements, Ge NCs had identical concentrations to absorption measurements. Samples were excited between 300-420 nm in 20 nm intervals. For excitation spectra, samples were probed at the emission maximum. Emission and excitation measurements were recorded on a Shimadzu RF-5301PC spectrofluorophotometer using an excitation and emission slit width of 5 mm. For time resolved emission measurements, time correlated single photon counting (TCSPC) was done using commercially available Halcyone system from ULTRAFAST. The excitation wavelength was 340 nm provided by the TOPAS C unit pumped by the CLARK MXR CPA 2010 femtosecond laser. The fundamental beam output from the CLARK laser is 760-780 nm at a repetition rate of 1 kHz and pulse width of 150 fs. This is split and fed into the TOPAS to generate 340 nm pulses. The photoluminescence is collected by 2-inch diameter plane mirrors and collimated using a set of lenses to be guided to the detector. The detector is a PMT at the back of a double monochromator. The PMT is operated at a voltage of 2.25 kHz. The PL is attenuated by a set of neutral density filters to ensure that the data is collected at a rate of 25 counts per second (cps). The instrument response function was measured in the solvent without Ge NCs. The data was fitted with the curve obtained by convoluting the IRF with a single or double exponential model. RESULTS AND DISCUSSION


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Our surface chemistry modification strategy is shown in Scheme 1. Starting from assynthesized Ge NCs, 1-hexene, 1,7- octadiene, or allylamine are attached by UV induced hydrogermylation producing alkyl, alkene, or amine terminated Ge NCs. Carboxylic acid terminated Ge NCs were produced by thiol-ene click chemistry, reacting alkene terminated Ge NCs with 3-mercaptopropionic acid in the presence of UV light. The use of thiol-ene click chemistry has been previously utilized successfully in Si NC surface chemistry.60-61 The stepwise approach to carboxylic acid formation is important as attaching bifunctional molecules to Ge NCs can be difficult due to competing reactions between functional groups.62-63 This is particularly true when one end contains functionalities that are highly reactive towards Ge surfaces such as alcohols and carboxylic acids.62

Scheme 1 – Schematic representation of the surface chemistry used in this study. Hydrogermylation is used to covalently attach organic molecules to the surface (R = alkyl, alkene, amine). Thiol-ene click chemistry is used to terminate the Ge NCs with carboxylic acid functional groups.


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Figure 1 – Low-resolution TEM images of alkyl (a), alkene (b), carboxylic acid (c), and amine (d) terminated Ge NCs, and histograms of nanocrystal sizes for alkyl (e), alkene (f), carboxylic acid (g), and amine (h) terminated Ge NCs. Low-resolution TEM images of alkyl, alkene, carboxylic acid, and amine terminated Ge NCs are seen in Figure 1a, b, c, and d, respectively. Ge NCs are spherical in shape and relatively monodisperse with average sizes of 4.9 ± 0.9 nm (alkyl), 5.0 ± 0.9 nm (alkene), 4.6 ± 1.0 nm (carboxylic acid), 5.4 ± 1.1 nm (amine) (Figure 1e, f, g, h, respectively). The size and monodispersity of the Ge NCs agrees well with reports by Prabakar et al.53 highlighting the level of size control available from the halide salt reduction method. Importantly the sizes of Ge NCs are similar, enabling comparisons of the optical properties across distal groups.


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An electron diffraction pattern of alkyl terminated Ge NCs is shown in SI Figure 1a. The rings can be indexed to the (111), (200) and (311) spacings of crystalline germanium. SI Figure 1b and c is a high resolution TEM image of a 5 nm alkyl terminated Ge NC and its corresponding fast Fourier transform (FFT). The NC is highly crystalline with clearly resolved lattice fringes. The corresponding FFT shows that HRTEM image matches the germanium crystal structure when viewed down the [111] axis.

Figure 2 - FTIR spectra of alkyl, alkene, carboxylic acid, and amine terminated Ge NCs. (alkyl = purple, alkene = blue, carboxylic acid = orange, amine = red). Fourier transform infra-red (FTIR) spectra of alkyl, alkene, carboxylic acid and amine terminated Ge NCs are seen in Figure 2. Peaks at 738-743 cm-1 and 2938-2890 cm-1 (present in all samples) are characteristic of Ge–CH2 and C–H vibrational stretches.49 The lack of absorbance between 870-910 cm-1 suggest Ge NCs contains Ge-oxide composition. The small peak at 1619 cm-1 and the shoulder peak at 3030 cm-1 for alkene terminated Ge NCs indicates the presence of C=C and C=C-H bonds, respectively. Peaks at 1645 cm-1 and 3400 cm-1 for carboxylic acid terminated Ge NCs confirm the presence of the carbonyl (C=O) and hydroxyl (OH) groups. The approximate 50 cm-1 redshift observed here for a carboxylic acid carbonyl 11 ACS Paragon Plus Environment

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is likely due to this carbonyl group’s close proximity to the electron-withdrawing thiol group. The absence of peaks at 1619 cm-1 and 500 cm-1 in the carboxylic acid terminated Ge NCs also indicates the successful grafting of the mercaptopropionic acid without the formation of proximal Ge-S bonds. The Broad peak between 3700-3225 cm-1 and the small peak at 1648 cm-1 for amine terminated Ge NCs are due to N–H stretching and scissoring, respectively. The successful surface functionalization of Ge NCs was also confirmed by 1H NMR. 1H NMR spectra of alkane, alkene, amine, and carboxylic acid terminated Ge NCs is seen in SI Figure 2. The 1H NMR spectrum of alkane terminated Ge NCs shows 3 key peaks at 0.85 ppm (doublet), 1.14 ppm (multiplet), 1.24 ppm (doublet), which correlates to hydrogen atoms attached to the proximal methyl, β to Ge-methylene, and methylene chain, respectively (SI Figure 2a). The 1H NMR spectrum of the alkane terminated Ge NCs also shows a peak at 3.50 ppm (multiplet) matching the surfactant used in particle synthesis (TOAB).20 The 1H NMR spectrum of alkene terminated Ge NCs shows 2 key peaks at 4.92-5.10 ppm (triplet) and 5.71-5.82 ppm (multiplet) corresponding to the hydrogens on the terminal alkene group (SI Figure 2b). The 1H NMR spectrum of the alkene terminated Ge NCs also shows a peak at 3.50 ppm like the alkane terminated Ge NCs. The 1H NMR spectrum of carboxylic acid terminated Ge NCs shows 3 key peaks at 2.48 ppm (multiplet), 2.68 ppm (multiplet), 8.38 ppm (singlet) characteristic for hydrogen atoms adjacent to the thiol group and the hydroxyl group of the carboxylic acid (SI Figure 2c).17 The 1H NMR spectrum of carboxylic acid terminated Ge NCs does not contain alkene peaks (5-6 ppm) indicating complete surface modification. The 1H NMR spectrum of amine terminated Ge NCs shows 3 peaks at 3.5-3.6 ppm (doublet), 5.4 ppm (multiplet), 5.9 ppm (multiplet) which correspond to hydrogens adjacent to the distal amine groups and hydrogens attached to an alkene group, respectively (SI Figure 2d).


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X-ray photoelectron spectroscopy (XPS) was used to further determine NC composition and surface species. High-resolution X-ray scans of the Ge 3d and N 1s regions of alkane, alkene, carboxylic acid, and amine terminated Ge NCs are shown in SI Figure 3 and 4, respectively. The Ge 3d5/2 binding energies of the amine and carboxylic acid terminated NC were found at 29.8 eV and 29.7 eV, respectively. For the alkane and alkene terminated NC, the Ge 3d5/2 binding energy suits respectively at 31.3 eV and 31.9 eV. These results suggest that the synthesized Ge NC presents superficial redox states located between Ge0 and Ge2+.52 Figure S5d also reveals a peak 32.7 eV, which would match for Ge4+. However, this peak was found only for the carboxylic acid terminated NC, where the presence of Na was also revealed by XPS. As the binding energy of Na2p suits at the same region, we hypothesize that sodium is responsible for the peak at 32.7 eV in Figure S5d. The absence of a Ge3p1/2 peak above 128 eV also supports the absence of Ge4+. For all produced Ge NCs the N 1s region shows no evidence for Ge-N species, as no N 1s peak was found at 398 eV (SI Figure 4a-d).64 Analysis of the O 1s region (SI Figure 5) showed only signals corresponding to organic and adventitious oxygen (531.5 – 532 eV), rather than oxygen arising from a metal oxide. In conjunction with the FTIR data, this leads us to hypothesize that GeO levels are not significant in our sample. Analysis of the Ge3d5/2 region eliminates the possibility of GeO2. Therefore, we believe the possibility of oxide influence on the optical properties of the QDs is minimal. Also important is the observation that, for the carboxylic acid terminated Ge NCs, no evidence for Ge-S in either the Ge 3d region or the S 2p region (SI Figure 6 was observed,65 matching well with 1H NMR and FTIR data which indicated no Ge-S binding. The S 2p peak signal at 163.2 eV matches well with the signal corresponding to C-S-C.66 The S:COOH ratio revealed by XPS was 1:1, indicating that the structure of the molecule is preserved after immobilization. The signal for S2p was not attenuated, showing that a significant amount of sulfur is close to the outer surface.65 Additionally no Li signals were 13 ACS Paragon Plus Environment

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observed over any of the XPS scans for all Ge NCs. With Si NCs quaternary amine compounds like TOAB are known to bind to the surface, complicating iterative chemical reactions on Si NC capping molecules and Si NC biosensor applications.20 The highresolution XPS scans of the Ge 3d and N 1s region for alkane and alkene terminated Ge QDs do not show clear evidence for Ge-N species. This suggests that TOAB does not bind to the Ge surface, and that the signal corresponding to TOAB in the 1H NMR spectrum (Figure S4a) is likely due to free TOAB molecules associated to the Ge NCs post-purification rather than surface-bound. The weak affinity of ammonium compounds to Ge NC surfaces is a major advantage for Ge in their practical biomedical and optoelectronic applications compared to Si as TOAB concentrations as low as (< 0.2 % atomic percent of nitrogen) are known to modify the optoelectronic properties of Si NCs.20 The lack of Ge-N species in the XPS of the amine terminated Ge NCs (Figure S6) also suggests that allylamine is proximal bound via Ge-CH2 rather than Ge-NH2 concluded from the 1H NMR results earlier. The exclusive binding of allylamine through proximal Ge-CH2 is an exciting result for Ge NCs, completely different from Si NCs which show both proximal alkene and amine when allylamine is used.20 This result highlights that Si and NCs are not completely alike in their chemical reactivity and fundamental studies are equally required on both systems to unlock their full potentials.


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Figure 3 – Absorption (thick line), emission (thick line) and excitation (dotted line) spectra of alkyl, alkene, carboxylic acid, and amine terminated germanium nanocrystals (λex = 340 nm; alkyl = purple, alkene = blue, carboxylic acid = orange, amine = red). The absorption, emission and excitation spectra for alkyl, alkene, carboxylic acid, and amine terminated Ge NCs are seen in Figure 3. Alkyl, alkene, carboxylic acid, and amine terminated Ge NCs show increasingly broad absorption from the onset (~400 nm). Tauc plot analysis (SI Figure 7) gave an absorption onsets of 3.60 eV (344 nm), 3.63 eV (343 nm), 3.76 eV (330 nm), 3.55 eV (349 nm) for alkane, alkene, carboxylic acid, and amine terminated Ge NCs, respectively. The emission spectra (λexcitation = 340 nm) of alkyl, alkene, carboxylic acid, and amine terminated Ge NCs is also seen in Figure 3. Full emission spectra for all Ge NCs is seen in SI Figure 8. For non-polar distal groups, emission maxima were 412 nm (alkyl) and 423 nm 15 ACS Paragon Plus Environment

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(alkene). For polar distal groups the emission maxima were 510 nm (carboxylic acid) and 491 nm (amine), giving up to 98 nm redshift compared to non-polar distal groups. The Stokes shift was calculated by subtracting the difference in the absorption onset and emission maxima. The Stokes shifts of alkyl, alkene, amine, and carboxylic acid were 13 nm, 16 nm, 81 nm, and 95 nm. The excitation spectra of alkyl, alkene, carboxylic acid, and amine terminated Ge NCs are also shown in Figure 3. All Ge NCs have identical excitation maxima occurring at 338 nm. The absorption onset given by the Tauc plots in each case is higher in energy, >3.5 eV, compared to Ge NCs of similar size, ca 2.6 eV.40, 50 To obtain an accurate absorption edge from materials such as Ge which have both indirect and direct transitions can be challenging. This is particularly true in NC form where the oscillator strengths of transitions show a strong size dependence.67 We associate the difference between our observed absorption edges and previous literature values to the observation of different absorption transitions in Ge NCs. The absorption slope and onset of alkyl, alkene, carboxylic acid, and amine terminated Ge NCs are similar. Alkyl, alkene, carboxylic acid, and amine terminated Ge NCs also have very similar excitation spectra. Taking the absorption and excitation spectra together it suggests that absorption processes in Ge NCs are controlled by the NC core rather than proximal groups matching well with previous reports.42,

56

Regarding emission, band gap emission

from 4-5 nm sized Ge NCs occurs in the red-NIR region.39-40,

46

The observed emission

energy from Ge NCs produced here is too high to be occurring from the band gap and likely comes from direct bandgap transitions or unknown surface states. 51 In contrast to the absorption data, polar and non-polar surface distal groups had a strong effect on the emission maxima and Stokes shifts of Ge NCs. Increasing the polarity of the distal group resulted in emission redshifts of up to ~100 nm and controllable Stokes shifts of up to 171 nm. The differences in emission maxima and Stokes shift demonstrated by polar


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and non-polar distal groups cannot be accounted for by the proximal group as all Ge NCs have identical proximal groups. 45 nm – 75 nm emission redshifts in solution synthesized Si NCs were reported, when changing from non-polar to polar distal groups.17 The origin of these redshifts was hypothesized to come from the ability for polar distal groups to relax quantum confined states in the NC core.17 It is now known that blue/green emission in Si NCs is linked to the presence of SiOx bonds at the NC surface.20 We believe these observed redshifts in Si NCs are due to interactions between proximal SiOx bonds and distal groups. Comparatively the redshifts observed here likely come from interactions between Ge emissive states and distal groups i.e. a pure distal effect. The influence of the distal thiol group in the carboxylic acid-functionalized Ge NCs on the NCs’ emission properties is hypothesized to be minor. This hypothesis is built upon previous studies on the optical properties of Si NCs which have been functionalised using thiol-ene click chemistry.17 Cheng et al demonstrated that Si NCs with thiols within the alkyl chain had the same emission maxima as Si NCs passivated with just alkyl chains. 17 Tuning the Stokes shift of semiconductor NCs is particularly important as reabsorption can cause significant issues with emerging LED devices, luminescent solar concentrator (LSC) composites, and bioimaging probes.6, 58, 68 Although the largest reported Stokes shift here is less than half the magnitude of the “best in class” semiconductor NCs, we highlight that this Stokes shift is tunable.58-59, 69 Previous methods to tune the Stokes shift have been the use of dopants or the formation of type II core-shell semiconductor NCs, both of which can be synthetically challenging.70-73 Tuning the Stokes shift of semiconductor NCs through polarity of surface capping molecules provides a simple method to reliably control the Stokes shift without the formation of a core-shell structure or addition of dopants.


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Figure 4 - Quantum yield of alkyl, alkene, carboxylic acid, and amine terminated Ge NCs. (alkyl = purple, alkene = blue, carboxylic acid = orange, amine = red). To investigate how the polarity of distal groups affect the emission efficiency of Ge NCs, QY measurements were performed. QYs for alkyl, alkene, carboxylic acid, and amine terminated Ge NCs are seen in Figure 4. QYs were calculated using the comparative method developed by Williams et al.74 using quinine sulfate as a standard. The QY of alkyl, alkene, carboxylic acid, and amine terminated Ge NCs were 11.2 %, 10.7 %, 5.8 %, 7.1%, respectively.


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Figure 5 - a) Solvatochromism effects on emissions maxima of Ge NCs terminated with nonpolar surface capping molecules (alkyl and alkene) using a range of solvents (hexane, toluene, THF = tetrahydrofuran, CHCl3 = chloroform). b) Solvatochromism studies of Ge NCs terminated with polar surface capping molecules (carboxylic acid and amine) using a range of solvents (CHCl3 = chloroform, MeOH = methanol, DMF = N-dimethyformamide, water). (alkyl = purple, alkene = blue, carboxylic acid = orange, amine = red). The Ge QDs with hydrophobic and hydrophlic distal groups were soluble in hexane or water respectively, as shown in a colour photograph of Ge NCs in solution in SI Figure 9. To further understand the role of the NC surface in the optical properties solvatochromism studies were carried out. For non-polar distal groups (alkyl, alkene) the emission maxima were recorded in hexane, toluene, tetrahydrofuran, and chloroform. For polar distal groups (carboxylic acid, amine) the emission maxima were recorded in chloroform, methanol, N19 ACS Paragon Plus Environment

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dimethyformamide, and MilliQ water. The excitation wavelength used was 340 nm. Alkyl and alkene terminated Ge NCs demonstrated a 10 nm redshift in the emission maxima going from hexane to chloroform. Carboxylic acid and amine terminated Ge NCs demonstrated a 30 nm and 20 nm redshift going from chloroform to water, respectively. The presence of a solvatochromic shifts in produced Ge NCs further confirm that emissive states are on the surface/near the NC surface20 The magnitude of solvatochromic redshifts strongly depended on the polarity of the distal groups with polar groups showing up to a 3 fold increase in emission redshift compared to non-polar distal groups. This difference in solvatochromic redshifts is likely due to relative strength of interactions between distal groups and Ge emissive states. Polar distal groups, amine and carboxylic acid, will interact more strongly with Ge emissive states causing a larger redshift. Comparatively non-polar distal groups, alkane and alkene, will have weaker interactions with Ge emissive states leading to smaller redshifts. Polarity of distal groups are known to alter the energy levels of PbS NCs lending credence to this idea.70

Figure 6 – Fitted time-resolved emission spectra of alkyl, alkene, carboxylic acid, and amine terminated Ge NCs (alkyl = purple, alkene = blue, carboxylic acid = orange, amine = red). Intensity is plotted on a logarithmic scale. 20 ACS Paragon Plus Environment

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Additional insight into how the polarity of distal groups affects emission processes can be gained using time resolved emission spectroscopy. All produced Ge NCs required biexponential fits, with time constants and amplitudes summarized in SI Table 1. Fitted ime resolved emission spectra of alkyl, alkene, carboxylic acid, and amine terminated Ge NCs are seen in Figure 6. The raw time-resolved emission data and errors associated with the fitting are shown in SI Figure 10 and SI Table 1.From SI Table 1, we see that majority of emission from alkyl, alkene, carboxylic acid, and amine terminated Ge NCs occurs within 1 ns of excitation. SI Table 1 also shows that both emission components are longer for polar distal groups compared to non-polar. Longer emission lifetimes of polar distal groups increase the probability for non-radiative events to occur, matching well with the lower QYs of polar distal groups observed earlier. The difference in lifetimes for non-polar and polar distal groups provide further evidence for interactions between emissive states and distal groups. Changes to emission dynamics induced by surface capping molecules have been previously observed in PbSe NCs.75 In the case of PbSe the role of energy transfer between NC core and vibrational states in surface capping molecules was established.75 The differences observed between PbSe and Ge NCs show that surface capping molecules affect the optical properties of semiconductor NCs in different ways. Overall the ability for surface capping molecules to alter the emissive pathways in Ge NCs is likely down to the polarity of distal groups. Non-polar distal groups will have weak electronic interactions with emissive states in Ge NCs as shown by the similar emission lifetimes, emission maxima, Stokes shift, quantum yield, and small solvatochromic shifts. Polar distal groups can modulate the energy of Ge NC emissive states due to electronic interactions between the Ge core and polar distal groups, altering the emission maxima, emission lifetimes, quantum yield, Stokes shift, and larger solvatochromic shifts.


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CONCLUSIONS Ge NCs with non-polar or polar distal groups were produced with near identical sizes enabling comparison of the optical properties. The absorbance properties of Ge NCs did not depend on the polarity of distal groups. The emissive properties of Ge NCs did depend on polarity of distal groups. By rational changes to the polarity of distal groups the emissive properties could be altered, including, Stokes shift, emission maxima and lifetime. These results are fundamentally different to the optical behavior of not only similar semiconductors, Si, but other semiconductor systems such as CdSe, highlighting the need for fundamental study on a system by system basis. This research demonstrates a pure distal effect on the optical properties of semiconductor NCs, and it is expected the principles found here will stimulate similar research with other semiconductor systems. Study into differing functional groups on surface capping molecules, including distal and proximal groups, will be particularly important in the development of next-generation NC LEDs and bioimaging agents.

ASSOCIATED CONTENT Supporting Information. Electron diffraction pattern, high resolution TEM image and corresponding FFT, 1H NMR spectra, high resolution XPS spectra, Tauc plots, full emission spectra between 300 – 400 nm excitation wavelengths, and photographs of Ge NCs under 365 nm excitation are all shown in Supporting Information. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]


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Present Addresses †If an author’s address is different than the one given in the affiliation line, this information may be included here. Notes The authors declare no competing financial interest. REFERENCES

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