Direct Synthesis of CdSe Nanocrystals with Electroactive Ligands

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Direct Synthesis of CdSe Nanocrystals with Electro-active Ligands Yasser Hassan, Trevor Janes, Ryan D Pensack, Shahnawaz Rafiq, Peter M Brodersen, Mitchell A. Winnik, Datong Song, and Gregory D. Scholes Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01212 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 27, 2016

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Chemistry of Materials

Direct Synthesis of CdSe Nanocrystals with Electro-active Ligands Yasser Hassan1,

2, 4

, Trevor Janes1, Ryan D. Pensack3, Shahnawaz Rafiq3, Peter M.

Brodersen2, 5, Mitchell A. Winnik1, 2, Datong Song1, and Gregory D. Scholes*,1 1

Department of Chemistry, 80 St. George Street, University of Toronto, Toronto, Ontario, M5S 3H6 Canada. 2 Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada, M5S 3E5. 3 Department of Chemistry, Princeton University, Washington Rd, Princeton NJ 08544 USA 4 Chemistry Department, Faculty of Science, Zagazig University, 44511 Zagazig, Egypt 5 Surface Interface Ontario, Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada, M5S 3E5.

ABSTRACT. We report the synthesis and characterization of cadmium selenide nanocrystals with electro-active ligands directly attached to the surface. The conventional surfactant-assisted synthesis yields nanocrystals with surfaces functionalized with insulating organic ligands. These insulating ligands act as a barrier for charge transport between nanocrystals. Electro-active (reducing/oxidizing) ligands like ferrocene and cobaltocene have potential for applications as photo-excited hole conductors and photoredox systems. Although ferrocene ligands anchored to the nanocrystal surface through insulating long-chain hydrocarbon spacers have previously been reported, this approach is limited because the charge transfer between nanocrystal and ferrocene is highly sensitive to their separation. We report here ferrocene directly bound to the inorganic core of the nanocrystal and as a result the distance between the nanocrystals and the electroactive moiety is minimized.

1 ACS Paragon Plus Environment


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TOC Graphic: CdSe nanocrystals are prepared directly with their surfaces passivated by electro-active ligands like ferrocene phosphine and phosphinoxide derivatives.


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1. INTRODUCTION Owing to their tunable physical properties that come from the quantum confinement effect, colloidal inorganic nanocrystals (NCs) or quantum dots have been examined for a diverse range of applications that include optoelectronic devices,1,

2

hydrogen reduction photocatalytic

processes3, 4 and biosensors.5-7 Facile transport of charge carriers between individual NCs is an essential requirement in optoelectronic devices based on these materials.8 In addition, the removal of photo-generated holes from the surface of the NCs, i.e., post photo-oxidation, was found to be essential to obtain efficient devices.9 In the conventional surfactant-assisted synthesis of NCs, surfactants need to be anchored to the NCs’ surface during their growth.10-12 These long chain organic surface capping ligands are important to control the NC growth, to prevent aggregation, to maintain the desired size, and to passivate surface electronic trapping states in the NCs.10-15 Moreover, the solubility of the semiconductor NCs is dictated by these capping ligands. A challenge, however, is that this conventional surfactant-assisted synthesis yields NCs with surfaces functionalized with insulating organic ligands. These insulating ligands act as a barrier for charge transport between NCs.8, 16, 17 It has been proven by Kuno et al. that complete removal of surface ligands is difficult and can create surface dangling bonds and charge-trapping centers that open up pathways for nonradiative relaxation18. However, an important feature of these surfactants is that they are easily exchanged on the surface of the NCs. In addition, it has been reported that in situ synthesis of NCs in a polymer matrix improves the polymer-nanoparticle interface, and consequently facilitates efficient electronic interaction between the polymer and NC.19-23 As reported previously,10-13 controlling the nucleation and growth process during NC synthesis is essential to obtaining monodisperse materials and this



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parameter is dramatically affected by the bonding strength and steric hindrance effects of the ligands as well as the presence of non-coordinating media.12 This hypothesis is found to be applicable in the case of the in situ synthesis of nanocrystals in polymers.19, 20, 22 In deposited films, electronic coupling between NCs, and consequently the charge carrier mobility, can be enhanced by removing the organic surfactants from the NC surface.24-26 The introduction of other competing ligands, through surfactant exchange, makes it possible to access a wide range of chemical functionalities.27 Ligand exchange has been used to adapt the surface of NCs for specific applications.8, 9, 25, 28 It was found that the chemical nature of the linker or the new ligand plays a decisive role in determining the efficiency of charge transfer from the NC surface to the surrounding media. An ideal ligand then, should be highly conducting, provide colloidal stabilization, and facilitate stable and facile electronic interactions between the NCs.8 Kovalenko et.al. introduced highly promising metal chalcogenide surface ligands in a ligand exchange process that increased the conductivity of NC solids (σ) by ~11 orders of magnitude compared to the ordinary organic ligands, approaching σ values of ( ~200 S cm−1).8 Electroactive ligands (reducing/oxidizing species) like ferrocene9, 26, 29-33 and cobaltocene34 have been studied for applications that include photo-excited hole conductors and photoredox systems. In previous reports, ferrocene was anchored to the surface of the NCs through hydrocarbon spacers with alkyl chains length of (3–12 carbon units).9, theory and simulations,35,

36

and experimentally31,

33

30

It was suggested by

that the hole transfer rate from the

photoexcited NCs surface (donor) to the ferrocene moiety (acceptor) depends on the hydrocarbon spacer length. The donor-acceptor charge transfer rate constant decreases exponentially with increasing donor-acceptor bridge distance28,

33, 37

. In addition, complete removal of organic

ligands through ligand exchange processes is difficult.


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The structure of the ligand can dramatically affect the shape and properties of the semiconductor NCs. The optimum ligands should meet the conditions of (i) highly conducting, (ii) adhere chemically to the NCs surface, thereby providing colloidal stabilization, (iii) providing stable and facile electronic communication between the NCs as the distance between the donor and acceptor is short.8 In this study, we report the direct synthesis and characterization of CdSe NCs with electroactive ligands of ferrocene phosphine and phosphine oxide derivatives without an intervening alkyl chain spacer. These metallocenes are directly bound to the inorganic core of the NC and as a consequence the distance between the NCs and the electroactive moiety is minimized. Ferrocene phosphinoxide derivatives, in particular, were found to be promising candidates toward the goal of a direct synthesis of CdSe NCs with electro-active ligands meanwhile the alkyl side chain, tert-butyl, necessary for colloidal stability is attached.

2. EXPERIMENTAL SECTION General Methods. All procedures were carried out using standard Schlenk line techniques under oxygen-free conditions and nitrogen flow. 2.1 Materials. All chemicals were used as received without further purification. Cadmium acetate (98.0 %), cadmium oxide (99.99 %), selenium (100 mesh, 99.5 %,), 1-octadecene (90 %), were purchased from Sigma-Aldrich and used without further purification. All solvents used including chloroform, methanol, toluene and isopropanol were anhydrous. 2.2 Synthesis of the Electroactive Ligands. Di-tert-butylphosphinoferrocene (FcPtBu2) and Di-tert-butylphosphinylferrocene (FcP(O)tBu2) (Scheme 1) were synthesized according to the method of Xiao et al. 38



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2.3 Direct Synthesis of CdSe NCs in Ferrocene Derivatives. We prepared two batches of CdSe quantum dots with different sizes: In a typical synthesis of CdSe NCs 3.5 nm size; 0.3 g of FcP(O)tBu2 and 0.1 mmol of Cdacetate were mixed with 5 mL of ODE in a 50 mL three-necked flask. After the reaction flask was pumped under vacuum for ~1 h at 120°C, the solution was heated to 220~250°C under argon or N2 flow. At that point, the Se-precursor was added swiftly into the reaction. The Se solution, prepared by dissolving 0.012 g Se powder and 0.3 g of FcPtBu2 were mixed with 3 mL ODE at 70°C under N2. This Se-precursor degassed and stored under nitrogen at 70°C before it used. The reaction temperature was adjusted to 10 degrees lower than the injection temperature, and the reaction was stopped after several minutes by the removal of the heating mantle and the injection of anhydrous toluene. The growth CdSe NCs was monitored by taking aliquots from the reaction with interval of time (2, 5 and 10 minutes) and measuring absorption and photoluminescence spectra. The reaction solution color changed, almost after the first two minutes, from bright orange to dark brown indicating the formation of CdSe NCs. CdSe NCs 7 nm size. In order to increase the size of the particle, we increased the concentration of the both cadmium and selenium precursors and added the selenium precursor drop-wise in two additions. Typically, the cadmium precursor was prepared by mixing 0.55 g of FcP(O)tBu2 and was dissolved in 7 mL of ODE and mixed with 0.045 g of Cd-acetate in a 50 mL three-necked flask. The reaction flask was pumped under vacuum for ~1 h at 120°C and the solution was heated to 220~250°C under N2 flow. The mixture was further held under vacuum for 10 minutes before the flask was allowed to return to 220°C under nitrogen. At that point, the Se-precursor (3 mL) was added drop-wise into the reaction at a rate of 0.4 mL/min. After 10 minutes from the first injection, the rest of the Se-precursor (4 mL) was added drop-wise into the reaction. The


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reaction temperature was adjusted to 220–250 °C, and the reaction was stopped after several minutes by the removal of the heating mantle and the injection of anhydrous toluene. The growth of CdSe NCs was monitored by taking aliquots from the reaction with interval of time (10, 15 and 30 minutes) from the second injection. The Se solution, prepared by dissolving 0.036 g Se powder and 0.256 g of FcPtBu2, was mixed with 7 mL ODE at 70°C and degassed under N2 before it used. Nanocrystals were allowed to grow at 220 °C. It is worth mentioning that the inverse-injection technique works well for this system. Typically, the Cd-precursor, once prepared at 220–250 °C was cooled to 50 oC before it was injected to a hot solution of Seprecursors at 220–250 °C. We found that the Se-precursor was more stable at high temperature than the Cd-precursor. The NCs were isolated and cleaned by several precipitation and redissolution cycles using toluene or chloroform as solvent and acetonitrile as the non-solvent (1:3 solvent to non-solvent ratio). Precipitation was achieved by centrifugation for 10–20 min under 6000 rpm. As a comparison, we also prepared CdSe according to Cao and coworkers method39 using cadmium acetate and selenium powder in ODE at 240°C to reach the desired size (3nm particles). Post-synthesis, we stabilized the NCs in FcP(O)tBu2 ligands by performing ligand exchange. 2.4 Characterization. TEM, HRTEM and EDX. Size distributions and energy-dispersive X-ray spectra of the prepared CdSe NCs were determined by transmission electron microscopy (TEM) using a JEOL JEM– 2010 TEM with a LaB6 filament operating at 200 kV for both low-magnification (TEM) and high-resolution (HRTEM) images.



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SAED. Selective Area Diffraction pattern (SAED) for collection of CdSe NC particles deposited on carbon grid were characterized using transmission electron microscopy (TEM; JEOL 2010 at 200 kV). SAED were simulated using the open-source software: (GDIS40 and Diffraction Ring Profiler41), with the input of known space group information of the CdSe hexagonal phase (Wurtzite).42,

43

The diffraction ring profiler integrates the SAED ring pattern intensities to

accurately calculate the center point of each ring. UV-vis Absorption and PL spectra were recorded using a Varian Cary 100 and Varian Eclipse fluorescence spectrometer, respectively, in a 1 cm cuvette. The PL recorded with excitation wavelength of 450 nm. Powder X-ray diffraction (PXRD) measurements were carried out on a SiemensD-5000 diffractometer using a high-power Cu Kα source operating at 50 kV and 35mA with a Kevex solid-state detector. A step scan mode was used for data collection with a step size of 0.028 and time of 2.0 s per step. X-ray photoelectron spectroscopy (XPS) data was acquired on a ThermoFisher Scientific KAlpha spectrometer with a monochromatic Al Kα X-ray radiation source in an ultrahigh–vacuum chamber with base pressure of 10−9 Torr. The generated X-ray photons is 1486.7 eV in energy. XPS analysis were performed on samples drop-casted onto Si(100) substrates whereby both survey and regional spectra were acquired from all samples measured. All data analyses were carried out using the Avantage software fitting program (provided from the vendor of the instrument) to confirm the incorporation of Cd, Se, P and Fe in the CdSe NCs. In order to investigate the chemical state(s) present in each sample; the elemental composition was calculated and the experimental data was mathematically modelled to deconvolute the peak shapes. The peak shapes were deconvoluted utilizing Lorentzian-Gaussian peak shapes and smart


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background function to approximate the experimental backgrounds. Surface elemental compositions were calculated from background-subtracted peak areas derived from transmission function corrected regional spectra. Scofield Al K-alpha sensitivity factors were used to calculate the relative atomic percentages. The binding energies were referenced to the NIST-XPS database. Cyclic Voltammetry. CV scans were run using a BASi Epsilon Electrochemical Workstation in a nitrogen-filled glovebox. A 3 mm diameter glassy carbon disk, a platinum wire, and a silver wire were used as working, counter, and pseudo-reference electrodes, respectively. Experiments were conducted in dry, deoxygenated dichloromethane; 0.1 M NBu4PF6 (recrystallized from EtOH) was used as supporting electrolyte. CV scans were obtained without compensation for internal resistance. 1,2,3,4,5-pentamethylferrocene (Me5Fc) was synthesized according to a previously published procedure.44 Potentials were calibrated using ferrocene as internal standard unless otherwise stated and are reported vs. Fc/Fc+. A cyclic voltammogram of a 0.7 mM solution of FcP(O)tBu2 and FcP(O)tBu2 recorded at a scan rate of 20 mV/s is shown in (Figure 6a). Ferrocene was added as an internal standard in the case of FcP(O)tBu2, for FcPtBu2, Me5Fc was used as an internal reference to minimize overlap of the peak potentials with ferrocene. NMR. 1H and 31P NMR spectra were recorded at ambient temperature on VARIAN Mercury 300 MHz and AGILENT DD2 600 spectrometer (600 MHz for 1H and MHz for the 31P) with 5 mm OneNMR H/F{X} probe. Chemical shifts were referenced to deuterated solvent peaks).

3. RESULTS AND DISCUSSION Electroactive Ligands. In this work we demonstrate a direct synthesis approach yielding high-quality CdSe NCs with electro-active ligands. This method includes synthesis of precursor materials in mild and simple reaction conditions. In a typical synthesis, the electro-active ligands 9 ACS Paragon Plus Environment


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include both di-tert-butylphosphinylferrocene (FcP(O)tBu2) and di-tert-butylphosphinoferrocene (FcPtBu2)38 complexing ligands (Scheme 1) for cadmium and selenium precursors, respectively. We found that the direct synthesis with FcPO is important over post-synthesis ligand exchange. Because the electron transfer rates decrease exponentially with increasing donoracceptor distance,28, 33, 37 we selected these two ligands, FcP(O)tBu2 and FcPtBu2 to ensure the short NC’s surface-to-ligand distance. X-type ligands45 like ferrocene monothiol, 1, 1′-ferrocene dithiol, ferrocene monocarboxylic acid, 1, 1′-ferrocene dicarboxylic acid can be used as short chain conductive ligands and provide better stability. For example, we carried out ligand exchange experiments for CdSe NCs post synthesis in trioctyl phosphinoxide system (TOPO, 90 %) using other short chain ferrocene derivatives like ferrocene monothiol, 1, 1′-ferrocene dithiol, ferrocene monocarboxylic acid, 1, 1′-ferrocene dicarboxylic acid. These experimental tests showed that ligand exchange resulted in the aggregation of the NCs and their precipitation out of solution. On the other hand the direct synthesis with L-type ligands45 of FcP(O)tBu2 and FcPtBu2 is important for several reasons. Though our L-type ligands might not provide adequate stability due to weaker interaction of P-atom with the NCs surface, the presence of an alkyl side chain provides some steric hindrance that controls the growth process and prevents aggregation in solutions. We synthesized the FcPtBu2 and FcP(O)tBu2 ligands according to the method of Xiao et al,38 then we carried out the post-synthesis characterization for these ligands, e.g., using NMR, and FTIR spectroscopies, to confirm the purity of the products. In order to examine the effect of the reaction temperature on FcP(O)tBu2 stability, a blank reaction of FcP(O)tBu2 in ODE without any added Cd or Se salts was carried out. Based on UV/Vis absorption spectroscopy, thin layer chromatography, thermogravimetric analysis (TGA), 10 ACS Paragon Plus Environment

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and differential scanning calorimetry (DSC) measurements, no degradation of FcP(O)tBu2 was observed until 280 °C (See Supporting Information; Figures S1–4). Therefore, we conducted the semiconductor NC synthesis at temperatures between 200 and 250 °C.

Scheme 1 Chemical structure of ligand used in this study (a) Di-tert-butylphosphinylferrocene (FcP(O)tBu2), (b) di-tert-butylphosphinoferrocene (FcPtBu2).

CdSe NCs Directly Synthesized with Electroactive Ligands. In a typical synthesis, we used 1octadecene (ODE) as a high-boiling point non-coordinating solvent. The Se-precursor of selenium powder coordinated by FcPtBu2 was introduced in a stoichiometric amount with respect to the Cd-precursor of cadmium acetate coordinated by FcP(O)tBu2 in ODE under inert condition at 200–250 oC. A quantitative analysis of the TEM images reveals that the resulting NCs are nearly monodisperse without any size separation. In order to understand the role of the ferrocene ligands during synthesis and the nature of the NC-ligand interaction, we used various methods to study CdSe capped with FcP(O)tBu2 NCs. These

characterization

methods

include

infrared

spectroscopy,

X-ray

photoelectron

spectroscopy, solution nuclear magnetic resonance spectroscopy (NMR). To investigate the charge transfer between the NCs and the ligands, we used photoluminescence spectroscopy, transient absorption spectroscopy and cyclic voltammetry.



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Shape Characterization. To check the morphology and chemical composition of the CdSe NCs capped with FcP(O)tBu2, several measurements were carried out such as

powder X-ray

diffraction (PXRD), energy–dispersive X-ray (EDX), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and the selected area electron diffraction pattern (SAED). TEM images of the synthesized NCs show crystalline spherical particles, Figure 1a and 1b. HRTEM analysis of CdSe NCs synthesized in ferrocene ligands shows well-developed lattice fringes revealing the (101) planes with a spacing of 0.35 nm (inset Figure 1b). HRTEM together with the selected area electron diffraction pattern (SAED) confirm the high crystallinity of the CdSe NCs sample. In order to examine the crystallinity of the NCs in more detail, we analyzed the TEM diffraction patterns (SAED) with Diffraction Ring Profiler software, which was developed for phase identification in complex microstructures.41 The SAED pattern of a collection of CdSe NC particles (in Figure 1c) exhibits broad diffuse rings that are typical of nano-sized particles and show crystalline particles with the patterns index of (100), (002), (101), (102), (110), (103), and (112) reflections of the hexagonal structure of CdSe.42, 43 Elemental analysis was performed to confirm the composition of the synthesized NCs using scanning electron microscopy energydispersive X-ray spectroscopy (SEM-EDX). EDX results, shown in Figure 1d, indicate traces of Cd, Se, Fe, and P elements in the CdSe capped with FcP(O)tBu2 composite sample. TEM studies revealed the abundant formation of nearly homogeneously dispersed spherical CdSe NCs.


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Figure 1. Low-magnification transmission electron micrograph (TEM) analysis for CdSe capped with capped with ferrocene derivatives (FcP(O)tBu2 and FcPtBu2); directly synthesized in FcPO(tBu2) at 250 °C with the size of (a) 3.5 nm (b) 7–7.5 nm. Inset: High-resolution electron microscopy (HRTEM), (c) Selected area electron diffraction pattern (SAED) of as-prepared CdSe together with the simulated electron diffraction patterns pattern of the CdSe hexagonal structure and (d) Energy dispersive X-ray (EDX) analysis of CdSe NCs (size: 7–7.5 nm).

Powder X-ray diffraction (PXRD) patterns of the free FcP(O)tBu2 ligand and the CdSe– FcP(O)tBu2 NCs reveal further details of the NC crystallinity and are presented in Supporting information, Figure S5 and Figure S8, respectively. CdSe NCs have been purified by iteratively precipitating in acetonitrile several cycles and subsequently re-dissolving in toluene in order to remove excess and weakly bound surface ligands. Strong reflection peaks at 23.89o, 25.39o, 27.09o, 41.99o and 49.73o, corresponding to the (100), (002), (101), (110) and (112) planes, respectively, indicate high crystallinity of a typical diffraction pattern of hexagonal Wurtzite CdSe phase (space group P63mc).42, 43 The main reflection peaks of CdSe at 25.39o, 35o, 41.99o



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and 50o are overlapped with peaks from FcP(O)tBu2 ligand crystalline phase, Figure S5 for comparison. The CdSe–FcP(O)tBu2 PXRD spectrum is in agreement with the SAED results (Figure 1c) for the same sample. The results of these characterizations of CdSe NCs directly synthesized in ferrocene derivatives (FcP(O)tBu2/FcPtBu2) are in good agreement with previous studies of CdSe NCs prepared with the conventional surfactant-assisted synthesis method. This confirms that we have pure CdSe NCs cores without doping with Fe atoms. Electroactive Ligand Attached to The Surface of the NCs: (i) Optical Properties. In the interest of characterizing the electronic structure of our NC, we studied their optical properties using UV–vis spectroscopy and PL measurements. Figure 2 shows the UV/Vis spectra of the electro-active ligands di-tert-butylphosphinyl ferrocene (FcP(O)tBu2) and CdSe–(FcP(O)tBu2) dissolved in toluene. The spectrum of FcP(O)tBu2 shows a broad peak in the range 400–520 nm with a maximum at 451 nm that is characteristic of the ferrocene moiety. For the solution of the CdSe–(FcP(O)tBu2) composite, the absorption spectrum exhibits a peak at 555 nm that we attribute to the first CdSe exciton peak, i.e. the s-like (1S3/2–1Se) transition of the CdSe NCs, as well as significant absorption at wavelengths shorter than 380 nm.28 A tail extending out to longer wavelengths than 555 nm is also apparent and we attribute this to NC aggregation in the solution that occurs over time. These features, absent in the case of FcP(O)tBu2 alone, indicate the presence of CdSe NCs. The ferrocene metal to ligand charge transfer (MLCT) band is evident in the absorption spectrum, somewhat overlapped with the nanocrystal (1P3/2–1Pe) band at around 488–503 nm. The absorption peak at 488 nm was noticed to be diminished after washing the nanocrysals with acetonitrile to remove ferrocene ligands from surface of the NCs, Figure S10.


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We observed no photoluminescence from the ferrocene-capped NCs, indicating that the excited state of the CdSe NCs is efficiently quenched (see the Supporting Information). This is attributed to either the rapid hole transfer to ferrocene or an increase in surface trap states due to the sterichindrance of the cyclopentadienyl rings of ferrocene on the surface of the NCs.

Figure 2. UV-vis absorption spectra of FcP(O)tBu2 in toluene (yellow), CdSe NC (synthesized in FcP(O)tBu2) in toluene (red). The ferrocene absorption peak associated with the CdSe NCs is red shifted from the original pure ligand by 0.28 eV.

(ii) Surface Characterization. To investigate the nature of the surface of the NCs, we carried out several measurements such as Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and nuclear magnetic resonance (1H and

31

P-NMR)

measurements. FTIR spectra of the FcP(O)tBu2 ligand and FcP(O)tBu2 capped CdSe quantum dots are presented in Figure 3. Infrared transition energies are sensitive to both chemical composition and structural environment. FTIR spectra of the free unattached FcP(O)tBu2 ligand, blue line, exhibit (C–H)Fc sp2 stretching, aromatic (C–C)Fc stretching, (C–H)Fc in plane bending and (C–H)Fc out of plane bending vibrations at 3089 cm-1, 1476 cm-1, 1022 cm-1 and 815 cm-1, respectively, which are associated with the aromatic rings of ferrocene moiety.46, 47 The vibrations at 2950 cm-1 and 2867 15 ACS Paragon Plus Environment


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cm-1 are assigned to νasym(C–H) and νsym(C–H) of the tert-butyl side chains, on the other hand, the vibrations at 1369 cm-1, 1144 cm-1 and 1108 cm-1 are assigned to the P–CH3 bending and P=O symmetric and anti-symmetric stretching absorption bands of the phosphinoxide moiety, respectively. FTIR spectra of the CdSe NCs capped with FcP(O)tBu2 ligand (green line in Fig. 3), show that the P=O stretching of the FcP(O)tBu2 ligand is broadened, split and downshifted in the range of 1000–1110 cm-1 relative to the free ligand which is attributed to complexation with the CdSe surface.48, 49 This is in agreement with previous IR measurements performed on P=O stretching frequencies of complex-formed by phosphine oxides with salts.50

Figure 3. FTIR transmittance spectra: FcPO(tBu2) (blue line) and CdSe NCs dispersed in KBr (green line). The characteristic sym. and anti-sym. absorption bands of P=O stretching of FcP(O)tBu2 were shifted to lower wavenumber and broadened indicating complexation between the P=O moiety and the CdSe surface.

Multiple samples of CdSe quantum dots capped with FcP(O)tBu2 were examined by X-ray photoelectron spectroscopy (XPS) in order to study the change in composition and chemical bonding at the surfaces of CdSe NCs and the capping ligand. Figure S11 shows a typical low resolution XPS full survey scan of CdSe NCs (6–7 nm) capped with FcP(O)tBu2. The presence of Si is from the substrate, while the quantitatively significant peaks for Cd, Se, P and Fe are from 16 ACS Paragon Plus Environment

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the NCs and their surfaces. The presence of C and O might be from the CdSe NCs surface and/or from atmospheric contamination. Figure 4 shows high-resolution XPS spectra taken of the Cd, Se, P and Fe regions of CdSe NCs deposited from methanol solution. Several features can be assigned by analyzing the binding energy measured from these regions. Typically, one can define the chemical environment of each species on the surface of the NCs and their ratios. XPS indicated a binding energy of core level Se 3d peak at 55.08 eV (Fig 4a) and Cd 3d5/2 peak at 405.62 eV (Fig. 4b) and the difference between their peak positions is about 350.6 eV that is in agreement with the binding energy of bulk CdSe (Ref.48 and references therein).48,

49

Data for different samples show the

reproducibility of the results and show no shake-up peaks in the region of Cd and Se peaks. In order to determine the stoichiometry and chemical environments present in these NCs, their peaks were fitted according to a Gaussian-Lorentzian peak shape along with a Smart background. We also determined the area under the peaks,51 calculated the ratio of Cd:Se, and found the ratio to be 1:1 in all samples measured.



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Figure 4. XPS data for Cd 3d, Se 3d, P 2p and Fe 2p core levels, respectively, highlighting their transitions.

Moreover, we have investigated the XPS spectra of both Cd 3d5/2 and Se 3d for the two samples we prepared in this study of CdSe with size of 3 and 7 nm using identical experimental conditions. The results indicate three main characters in both Cd 3d5/2 and Se 3d: (a) broadening increased with decreasing particle size, (b) increasing the CdSe NCs size from 3 to 7 nm results in shifting to lower binding energy side, for instance the Cd 3d5/2 core level shifted from 405.62 eV for size of 3nm to 404.98 eV for size of 7nm, and Se 3d shifted from 55.08 eV for size of 3nm to 54.68 eV for size of 7nm as well as (c) an increase in the cumulative intensity of both the Cd 3d5/2 and Se 3d core levels lines.


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It has been reported previously that the size dependent broadening of the XPS spectra is due to the surface reconstructions or “atomic displacements” at the surface in the nanocrystals that cause structural disorder within the nanocrystals.52 However, the red-shifts of the core photoemission peak (e.g. Cd 3d5/2 and Se 3d in our case here) toward higher binding energy with decreasing the size of the nanoparticles have been proposed to be caused by quantum confinement effects and increasing the number of lattice strain in small clusters.53-58 Data for different samples show the reproducibility of the results and show no shake-up peaks in the region of Cd and Se peaks. In order to determine the stoichiometry and chemical environments present in these NCs, their peaks were fitted according to a Gaussian-Lorentzian peak shape along with a Smart background. We also determined the area under the peaks,51 calculated the ratio of Cd:Se, and found the ratio to be 1:1 in all samples measured. On the other hand, high-resolution scans of the P and Fe regions of the NCs that provide information about the surface of the NCs, were carried out to confirm the presence of ferrocene molecules attached to the surface of CdSe NCs. Moreover, the surface bonding interactions can be examined by XPS analysis. Figures 4c and d shows the XPS spectrum for the P 2p and Fe 2p regions, respectively, of CdSe NC samples and of the free ligands, for the reason of comparison. The XPS results of the P 2p peak position of the CdSe NC different samples ranged from 132.29 to 132.94 eV, with a normal distribution centered at 132.63 eV. This value is shifted by ∆ = + 0.6 eV higher than the P-peak position of the FcP(O)tBu2 free ligand with 132.6 eV. This small shift (0.6 eV) suggests weaker surface chemical interactions between the phosphinoxide (P=O) group attaching ferrocene to CdSe NCs. We attribute this to the electron withdrawing nature of ferrocene that results in decreasing the electron density at the P=O group making it less polarized.



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Comparing the shape of the P 2p peak in both free ligand and the CdSe NCs surface, the P 2p peak in the free ligand gives multiple head peaks while the P 2p peak acquired from CdSe samples surface have one smoothed peak. This is due to free P=O bonds in case of free ligand while on the other hand the P=O moiety is bound to the surface of CdSe NC. Because we use two ligands in our synthetic technique, FcP(O)tBu2 and FcPtBu2, we expect two types of P compounds that can appear on the surface of our NCs, P=O–Cd as in case of FcP(O)tBu2 ligand and/or P=Se from FcP(tBu2) ligand. However, no other P=O–Cd (from FcP(O)tBu2 ligand) chemical state was found in the XPS spectrum. One possibility is that the FcPtBu2 is oxidized during or after the synthesis to FcP(O)tBu2 as in the case of the TOPO/TOP system.48, 51 Figure 4d shows the XPS examination spectrum for the Fe 2p3/2 and 2p1/2 regions for the free ferrocene ligands, both FcP(O)tBu2 and FcPtBu2, and ferrocene on the surface of CdSe NC samples. In the case of the free ligands, Figure 4d, the XPS spectra give two peaks, located at nearly 708.69 eV, and 721.5 eV assigned for the ferrocene Fe 2p3/2 and 2p1/2 transitions respectively.59, 60 On other hand, in case of CdSe NCs samples, alongside these two peaks an additional predominant two broad peaks appeared at 711.4 and 725 eV. According to previous reports, these two peaks are characteristic of Fe(III) oxidation state derived from ferrocene ligands redox to ferrocenium molecules.59, 61, 62 The formation of ferrocenium molecules may be due to the photoexcitation of CdSe followed by a charge transfer between the ligand and the core NCs. As shown in Figure 4d, while the free ligands do not show the FeIII oxidation state, the CdSe–FcP(O)tBu2 NCs sample exhibits the FeIII peak. Moreover, washing the CdSe–FcP(O)tBu2 NCs sample from excess unreacted ferrocene ligands increased the intensity of the FeIII peak which supports our hypothesis that the FcP(O)tBu2 ligand is oxidized by CdSe. These results


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confirm that the FcP(O)tBu2/FcPtBu2 ligands were successfully capped on the surfaces of CdSe quantum dots in addition to the existence of charge transfer between the ligands and CdSe. 1

H and

31

P-NMR measurements were used for further investigation of the nature of the

inorganic-organic interface in the CdSe capped with FcP(O)tBu2 /FcPtBu2 ligands system. We examined the 1H and

31

P-NMR spectra of solution state of CdSe NCs and the free ligands in

deuterated solvents for comparison. Figure 5a shows the 1H NMR of the free unbounded FcP(O)tBu2 ligand; (300 MHz, CDCl3) δ 4.45 (m, 2H), 4.45 (m, 2H), 4.28 (s, 5H), 1.28 (d, J = 13.5 Hz, 18H). The two strong peaks at ∼4.45 ppm corresponding to Fc aromatic protons in the case of the bound ligand are interesting to examine (Figures S1 and S2 for more details). 1H NMR of CdSe–FcPO(tBu2); (600 MHz, d-toluene) in Figure 5b shows that sharp resonances in 1

H NMR spectra of free ligands are broadened due to the strong bond to the surfaces of NCs

which is in agreement with previous reports.9,

51, 63, 64

Due to the strong absorption of 1-

octadecene’s double bond in a region similar to the ferrocene moiety, we identified the peaks of 1-octadecene as contaminants from the synthesis and then suppress them using solvent suppression in 1D proton NMR technique. We speculate that the broadness of the ferrocene 1

HNMR band in Figure 5b to come largely from restriction of the rotation about the centroid-

metal-centroid axis in ferrocene when bound to the CdSe NC. 31P-NMR spectra of solution state of CdSe NCs shown in Figure S12, shows two peaks at 58.04 ppm for FcPO(tBu2) ligand and 13.97 ppm for the FcP (tBu2)-Se. However, the FcPO(tBu2) peak is predominant on the surface of the particles.



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Figure 5. 1H NMR spectra of free and bound ligands with chemical shifts indicated in ppm. (a) 1HNMR of free FcPO(tBu2) ligand in CDCl3. (b) ) 1HNMR of bound FcPO(tBu2) ligand in d-toluene; the peaks of 4.77, 4.82 and 5.48 ppm correspond to the protons of the ferrocene cyclopentadienyl rings.

Figure 6. Cyclic voltammogram: CV response of (a) FcP(O)tBu2 and FcPtBu2 free ligands and (b) CdSeFcP(O)tBu2 and CdSe-TOPO NCs.


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(iii) Cyclic Voltammetry. Cyclic voltammetry (CV) is an electrochemical technique in which the current-potential (i vs E) curves at well-defined scan rates are recorded. By measuring the CV for the NCs, two main pieces of information can be gained; the ionization potential Ip and the electron affinity which is correlated directly to the oxidation and reduction potentials, respectively, represented in the cyclic voltammogram.65 CFcP(O)tBu2: The voltammogram of FcP(O)tBu2, solid line Figure 6a, displays a quasireversible oxidation, with peak separation, ∆Ep, = 115 mV. The formal reduction potential E1/2 was taken at the average of the oxidation and reduction peak potentials; E1/2 = 183 mV vs Fc. On the other hand, the dashed line in Figure 6a, for the FcPtBu2 in dichloromethane, E1/2 of Fc = 532 mV vs Me10Fc and E1/2 of Me5Fc = 273 mV vs Me10Fc.66 Using this relationship (E1/2 of Me5Fc = -259 mV vs Fc) peak potentials are reported vs ferrocene. The first, quasi-reversible (∆Ep = 142 mV) oxidation occurs at E1/2 = 50 mV vs Fc. At more positive potentials there are additional oxidative (Eo = 693 vs Fc) and reductive (Er = 469 mV vs Fc) features likely due to redox processes involving the nonbonding electron pair on phosphorus.67 CdSe capped with ferrocene. In order for hole transfer between the NCs and the anchored ligands to be favorable, the band energy alignment between the ligands and the NCs must be properly aligned9. Thus, we examined the voltammogram of CdSe–FcP(O)tBu2 NCs synthesized according to the method mentioned previously. Accordingly, a sample of CdSe–FcP(O)tBu2 NCs is washed from excess unreacted ferrocene ligands using 40,000 rpm centrifugation, and then transferred to a glovebox under inert conditions to exclude oxygen. The NCs were dispersed in toluene prior to the measurements. We recorded a cyclic voltammogram of a filtered 1.5 mg/mL solution of the CdSe–FcP(O)tBu2 NCs at a scan rate of 10 mV/s. Ferrocene was added after the experiment as a reference. Figure 6b shows a lone oxidative peak of CdSe–FcP(O)tBu2 NCs— 23 ACS Paragon Plus Environment


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i.e., the ionization potential—at approximately 368 mV versus Fc; there is no corresponding reductive feature. This irreversible redox reaction could be presumably due to the consumption of electrons in a chemical reactions between the FcP(O)tBu2 ligands and the NCs whereby a hole transfer takes place.30 On the other hand, the CdSe capped with TOPO with the same size measured in our lab gives an oxidation peak at 881 mV versus Fc. As

previously

mentioned,

the

CdSe–FcP(O)tBu2

NCs

exhibit

no

detectable

photoluminescence (PL) emission. The PL quenching in CdSe–FcP(O)tBu2 NCs can be due to either the dissociation of photo-generated excitons by hole transfer from CdSe to the Fc ligand or to the population of defects at the CdSe NCs surface.16 An examination of the excited-state dynamics in CdSe–FcP(O)tBu2 NCs using femtosecond (fs) transient absorption (TA) spectroscopy supports the former decay pathway (see Supporting Information for experimental details).

Figure 7. Transient absorption of ligand-exchanged CdSe–FcP(O)tBu2 and CdSe–OA (red) NCs. (a) Time-averaged transient absorption spectra of ligand-exchanged CdSe–FcP(O)tBu2 NCs at delays of 1.5 and 260 ps. (b) Time-averaged transient absorption spectra of CdSe–OA NCs at delays of 0.6, 1.5, and 260 ps. (c) Transient absorption kinetics of the excitonic bleach of ligand-exchanged CdSe–FcP(O)tBu2 (orange) and CdSe–OA (red) NCs. Both kinetics are nonmonoexponential. The CdSe–OA NCs exhibit a weighted-average decay of ca. 1 ns whereas the ligand-exchanged CdSe–FcP(O)tBu2 NCs exhibit an initial rapid decay of ca. 2 ps.

To examine the nonradiative decay pathways in CdSe–FcP(O)tBu2 NCs following photoexcitation, we monitored the excited-state dynamics using femtosecond (fs) transient absorption (TA) spectroscopy. We also investigated the excited-state dynamics of CdSe NCs 24 ACS Paragon Plus Environment

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capped with either oleic acid (OA) for comparison. The samples were dispersed in chloroform for the measurements. Figure 7c displays the kinetics of the excitonic bleach feature of the CdSe–FcP(O)tBu2 CdSe–OA NCs. The kinetic traces of the excitonic bleach of the three samples, shown in Figure 7c, were fit with a sum of multiple exponential functions. The weighted-average exciton lifetime as obtained from fitting of the transient absorption signal for the OA capped NC systems is 1 ns. The decay of the bleach signal of the FcP(O)tBu2 NCs is more complex and shows an initial time constant of ca. 2 ps in addition to several longer time components. The shortest time component contributes most significantly to the overall decay of the bleach feature. The longer time components may be attributed to residual aggregated NCs present in the sample as noted above. Although the bleach does not give us a direct indication of hole transfer68-70, the hole transfer is independently confirmed by the induced absorption of ferrocenium cation peaking at ca. 580 nm in the TA data (Figure 7a). The photoinduced absorption signal was assigned to ferrocenium cation based on: (i) the reported absorption spectrum of ferrocenium cation shows its maximum at ca. 600 nm71, 72 and (ii) comparison of TA measurements of OA-capped CdSe QDs that do not show any induced absorption signal in this spectral range (Figure 7b). Based on the observation of the ferrocenium cation induced absorption in the transient spectrum obtained at ca. 1 ps, these data indicate that the hole transfer occurs within ca. 1 ps. The decay of the bleach and photoinduced absorption signal after 1 ps suggests an electron-hole recombination (Figure 7a,c).

4. CONCLUSIONS We reported the synthesis and characterization of CdSe NCS with electro-active ligands directly 25 ACS Paragon Plus Environment


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attached to the surface. In order to ensure a short surface-to-ligand distance, we used ferrocenebased ligands containing phosphine and phosphinoxide functional groups without an intervening alkyl chain spacer. We characterized the morphology of the NCs using TEM and XRD and found that the NCs are highly monodisperse and crystalline. Optical, NMR, and XPS spectroscopies confirmed that the electro-active ligands chemically adhere to the NC surface through the favored phosphinoxide functional group. In order to gauge their potential as a photo-excited hole conductor and photoredox system, we performed cyclic voltammetry on the ferrocene-capped NCs and found that the electronic structure is appropriate for hole transfer from the nanocrystalline core to the electro-active ligand. Complete fluorescence quenching is consistent with this picture and suggests a high rate of hole transfer from the NC to the electro-active ligand. These results suggest that the ferrocene-capped NCs represent promising photo-excited hole conductor and photoredox systems.

ASSOCIATED CONTENT Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional characterization for the free ligands in Figures S1–S6 and additional characterization for the CdSe–FcP(O)tBu2 NCs, Figures S7–S15.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The Natural Science and Engineering Research Council of Canada (NSERC) is gratefully acknowledged for financial support. YH is grateful to J Owen and E. Cassette for helpful discussions. The authors are grateful to N. Coombs for help with electron microscopy and S. Petrov for carrying out the PXRD measurements.

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Direct Synthesis of CdSe Nanocrystals with Electroactive Ligands

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