Chemical, Structural, and Quantitative Analysis of the Ligand Shells of

Subscriber access provided by University of Guelph Library

Review

Chemical, Structural, and Quantitative Analysis of the Ligand Shells of Colloidal Quantum Dots Adam J. Morris-Cohen, Michal Malicki, Mark D. Peterson, John W. J. Slavin, and Emily A. Weiss Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm302108j • Publication Date (Web): 30 Aug 2012 Downloaded from http://pubs.acs.org on September 9, 2012

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36

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

Chemistry of Materials

**Submitted for publication as part of the Special Issue on Synthetic and Mechanistic Advances in Nanocrystal Growth**

Chemical, Structural, and Quantitative Analysis of the Ligand Shells of Colloidal Quantum Dots Adam J. Morris-Cohen, Michał Malicki, Mark D. Peterson, John W. J. Slavin, and Emily A. Weiss Dept. of Chemistry, Northwestern University, Evanston, IL 60208-3113

Abstract: This review outlines the set of technical approaches to answering three major questions about the surface chemistry of colloidal semiconductor quantum dots (QDs): (i) What is the chemical structure of the ligands on the surface of the QD? (ii) How many of each type of ligand are on the surface of the QD? (iii) What is the intermolecular structure (geometry) of the ligands on the surface of the QD? Each section addresses the accessability of the relevant techniques – which include 1D and 2D NMR, vibrational and electronic absorption and transient absorption spectroscopies, and various elemental analyses – and their sensitivity and applicability to the specified observable of interest.

Keywords: quantum dot, NMR, elemental analysis, transient absorption spectroscopy.

1 ACS Paragon Plus Environment


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

Colloidal semiconductor quantum dots (QDs) are the quintessential hybrid material. Their crystalline inorganic cores are nanoscale chunks of bulk semiconductor, and their surfaces comprise a set of organometallic complexes. They are, therefore, complicated materials to characterize using the standard set of analytical tools optimized for either solid state or organic/organometallic chemistry. Quantum dots absorb light over a large range of wavelengths with high photon cross-sections, and these optical excitations produce delocalized charge carriers (electrons and holes) within the inorganic core of the QD. These properties themselves do not make QDs exceptional; they are exceptional because the electronic structure of the QD is uniquely situated between the electronic structures of bulk semiconductors and organic molecules. We, as chemists, can control the fate of the excitonic state of the QD so that the exciton may dissociate to create spatially separated charge carriers, as is the tendency of weakly bound excitons in bulk semiconductors, or recombine radiatively, as occurs for tightly bound excitons in organic molecules, in the absence of fast non-radiative pathways. We control the excitonic decay pathways by tuning the dielectric constant of the inorganic core, by adjusting the degree of confinement of the exciton, and by designing the surface of the QD to include or exclude relaxation channels for the photoexcited carriers. The first “knob” – the dielectric constant– is primarily a function of the material used for the inorganic core. The second two “knobs” – the confinement energy and the competition between radiative and non-radiative pathways – are tunable through the surface chemistry of the particle (confinement energy is also, of course, a function of the size of the QD). The importance of surface chemistry in dictating the behavior of photoexcited and injected charge carriers increases as the size of the particle decreases.


Page 2 of 36

Page 3 of 36

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


The surface chemistry of QDs is not, in general, simple. Quantum dots prepared by the majority of common synthetic procedures emerge from a complex mixture of impure reagents and unknown reaction kinetics.1 Many of these impurities are not inert; rather, they dominate the reactivity of precursors or serve as tighter-binding ligands than the intended surfactant.2-4 For example, we observed that the ratio of Cd to Se in CdSe QDs depends upon the concentration of tight-binding, alkylphosphonic acid impurities in the as-purchased coordinating solvent, trioctylphosphine oxide (TOPO); these impurities result in Cd2+ enrichment of QD surfaces by up to a factor of six.5 Buhro and coworkers showed that specific batches of commercially available TOPO yield different sample morphologies and growth rates for CdSe quantum wires due to small but variable amounts of impurities in commonly used reagents; these impurities are beneficial to controlled QD growth, albeit by an unknown mechanism.6 In current syntheses, there is often no discernible relationship between the stoichiometry of organic reagents and the final surface chemistry of the QDs. Analytical studies of the chemical composition and structure of QD surfaces and the influence of the impurities in the reagents are therefore critical in understanding their optical and electrical properties, and in developing syntheses that produce (and re-produce) homogeneous surfaces. In this review, we categorize the analytical work on the ligand shells of QDs by three major objectives in order of increasing technical difficulty: (i) identification of ligands adsorbed to QDs, (ii) quantification of ligands adsorbed to QDs, and (iii) characterization of the geometry and intermolecular structure of the ligand shell. In describing various approaches to these problems, we consider the cost and accessibility of the techniques, in order to guide researchers who are considering entering this field in deciding whether their instrumental resources are appropriate for their planned research program.



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

Identification of Ligands on the Surface of the Quantum Dot The most powerful and commonly employed method of identifying ligands on the surface of colloidal QDs is NMR spectroscopy. This technique is effective because (i) many ligands show a unique spectral signature that can be used to conclusively verify their presence in the system, (ii) the technique is non-destructive and can be used to analyze ligands in their native environment (and without perturbing the equilibrium between the bound and free states), and (iii) ligands in solution and on the surface of the QD can be distinguished by changes in linewidth and chemical shift within 1D NMR, Figure 1, and from the presence of specific coherences in 2D NMR. The sample preparation for NMR experiments is simple and data collection is fast, typically seconds to minutes. One of the main drawbacks in using NMR spectroscopy to characterize colloidal QD-ligand complexes is the need for relatively high concentrations of material required to achieve adequate signal when compared to other spectroscopic methods. While the range of concentrations needed for NMR analysis varies with the specific objectives of the experiment, we routinely prepare solutions of PbS QDs with concentrations of 10-4 M in order to obtain 1H NMR spectra within minutes of signal acquisition. 1

H NMR. With its ease of access and low cost of analysis per sample, 1H NMR spectroscopy

is a useful tool for characterizing the ligand shell on QD surfaces. There are many examples of the use of 1H NMR for the characterization of surface-bound thiols,7-9amines,10-12 phosphonic acids and phosphine oxides,3,13-15 carboxylic acids,13 and pyridine-containing ligands,10,16 on CdSe QDs; thiols on CdSe/ZnS QDs, CuInS2/ZnS, and PbS QDs;17-20 and alkylamines on ZnO QDs21. Importantly, 1H NMR probes all molecules present in the sample, and not only molecules that are in contact with the surface of QDs. Depending on the history and equilibrium state of the sample, it is possible that appreciable populations of both surface-bound and free ligands are


Page 4 of 36

Page 5 of 36

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


present in a sample. Distinguishing between these two populations is crucial in characterization of the ligand shell. 1H NMR resonances of protons within surface-bound species are broadened due to chemical-shift heterogeneity and/or decreased T2 relaxation time (Figure 1).10,13,22-26 For example, Shen et al. demonstrated that the relaxation time constant T2 of methyl groups in TOPO bound to CdSe QDs (2.7 nm diameter) is smaller by a factor of ten than that measured for free TOPO.26 Additionally, the 1H chemical shifts of organic molecules may be sensitive to adsorption due to changes in their molecular and/or supramolecular structure. For example, protons attached to the methylene group alpha to sulfur within decanethiolate shift downfield by more than 1.5 ppm upon adsorption to PbS QDs in toluene-d8.19 Changes in the 1H NMR spectrum of an organic molecule in the presence of QDs can also indicate a nanoparticlefacilitated chemical reaction, such as the photooxidation of thiols to disulfides in the presence of CdSe, CdS, or PbSe QDs.7,24 31

P NMR. Many of the procedures for synthesizing colloidal QDs use surfactants, such as

trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO), that contain phosphorous atoms. Although TOP and TOPO do not always remain adsorbed to the QDs after purification, tightbinding alkylphosphonate and phosphinate impurities in TOP and TOPO often dictate the growth dynamics of the QDs and serve as the final ligand shell of the QDs.27,28 In this case, and if the chemical shifts of each of the P-containing species are known, 31P NMR is an effective method for analyzing the composition of the ligand shell because the spectra are easy to interpret, often comprising a single peak or few peaks that do not overlap.29 Work by Wang et al.28,30 provides a good reference for the chemical shifts of many of the impurities found within the TOPO. Within a

31

P NMR spectrum, ligands bound to the surface of the QD can be distinguished

from those in solution through their chemical shift and linewidth. Early work showed that the



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

linewidth of peaks from phosphorous-containing ligands on colloidal QDs are broad due to the wide distribution of chemical environments at the surface of the QD.31 Koppinget al.2 developed a method to assign the broad resonances of surface-bound ligands in a 31P NMR spectrum: they first exchanged the native ligands on CdSe QDs with 4-(N,N-Dimethylamino)pyridine (DMAP) and benzyltrimethylammonium propionate, Figure 2. After ligand exchange, they were able to correlate the appearance of new, sharp resonances arising from the displaced P-containing ligands with the disappearance of the broad signal from the bound ligand. This work and other reports3,4 show that octylphosphonic acid (OPA) and the anhydride condensate of OPA are the predominant ligands in the capping layer of CdSe QDs synthesized in the presence of technical grade (90%) TOPO. 13

C NMR. Solution 13C NMR is not as sensitive as1H NMR spectroscopy and is rarely32 the

method of choice for characterization of the ligand shells of QDs. The broadening of signals originating from carbon atoms anchored to the QD surface – due to both reduction of the spinspin relaxation time and by the distribution of chemical shifts within the sample24,32 – is, however, useful for verifying adsorption of ligands to the surface. Advanced NMR techniques.In cases in which 1D 1H NMR spectroscopy does not allow one to distinguish between bound and free ligands, diffusion-ordered NMR spectroscopy (DOSY) and nuclear Overhauser effect spectroscopy (NOESY) are useful tools for proving attachment of organic molecules to a QD surface. 1H DOSY measures the diffusion coefficients of chemical species associated with observed 1H resonances, and therefore permits assignment of spectral features in the measured 1H NMR spectrum to different diffusional species.33 In the case of samples which contain a mixture of ligands adsorbed to the QDs and free ligands in solution, DOSY can differentiate spectra of these two types of ligands.13,18,21,26,34-36 For DOSY to


Page 6 of 36

Page 7 of 36

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


distinguish between surface-bound and free ligands the difference in hydrodynamic radii, RH, of the two species must be at least larger than the error of the measurement (we observe errors that are < 5% of the calculated hydrodynamic radius). Analysis of DOSY spectra is greatly simplified if the surface-bound ligands are attached to the surface of QDs for the duration of the experiment and do not undergo chemical exchange between bound and free states; in practice, this limit applies to systems in which the rate of exchange under equilibrium conditions is > 50 s -1.11,19,37 Gomes and coworkers used DOSY to measure RH of free oleic acid (0.56 nm) and of CdSe QDs after reacting the particles with oleic acid (OA) (4.2 nm) in THF.13 The clear difference between RH of surface-bound and free OA allowed the authors to conclude that reacting octadecylphosphonic acid (ODPA)-coated CdSe nanoparticles with OA at elevated temperatures leads to extensive ligand exchange (Figure 3).13 When NMR instruments capable of DOSY measurements are more accessible than high-resolution transmission electron microscopy, DOSY can be used to establish the average size of the nanoparticles.38 One drawback of DOSY is that the technique uses complex pulse sequences which include magnetic-field gradient pulses.33 Transfer NOESY (trNOESY) experiments are also useful for assigning 1H NMR resonances to molecules bound to the QD surface.11,13,35,37 The cross-coupling signals in trNOESY originate from dipole-dipole interactions among protons within the same molecule, but they are enhanced when the molecule is immobilized on a slowly tumbling object, such as a protein or a QD.11,21,37,39 NOESY also reveals coupling among ligands on the surface of QDs, Figure 1.19,36,40 We have used NOESY to demonstrate inter-ligand coupling between aminoferrocene and oleate on the surface of PbS QDs, even when the number of aminoferrocene molecules on the surface of QDs is as low as 5% of the total overall number of ligands.19 If trNOESYis inaccessible (for



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

example, due to the lack of a gradient coil), it is possible to test the influence of QDs on the dipole-dipole relaxation rate of the protons in the ligand of interest by comparing population inversion recovery times (T1 relaxation) for samples containing free ligands in solution and samples containing a mixture of ligands and QDs. Due to the dipole-dipole energy transfer, which is a manifestation of NOE,22 T1 relaxation times of protons in the ligand of interest are shortened if the ligand is in close proximity to other ligands on the QD surface. Shen et al. showed that T1 of methyl-group protons in CdSe-bound TOPO is half of the spin-lattice relaxation time of free TOPO in CDCl3 solution.26 We have demonstrated that Cp5 protons in aminoferrocene experience T1 values that are a factor of four shorter when aminoferrocene is bound to the surface of PbS than for free aminoferrocene.19 FT-IR and Electronic Absorption Spectroscopy. There are a number of examples in the literature describing the use of characteristic IR resonances of chemical groups to identify ligands present in QD samples.17,41-48 The technique is not as powerful as NMR for establishing whether the molecules are free or bound.43 It is important to note that the vast majority of FT-IR analyses are performed on thin film samples, where the solution-phase equilibrium has been perturbed. Nevertheless, considering its low cost, FT-IR is a good starting point for qualitative analysis of the ligand shell of QDs. The electronic and optical properties of QDs depend on their size, shape, and composition, and ground-state absorption spectroscopy is a quick and powerful tool for initial characterization of these materials.49,50 In addition to the size of the QD core, the degree of excitonic confinement (and therefore the absorption spectrum) depends on the symmetry and energy of the frontier orbitals in the capping ligands, which define the energetic barrier for tunneling of the excitonic wavefunction into the ligand shell. Spectral shifts in the first excitonic peak after ligand


Page 8 of 36

Page 9 of 36

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


exchange can therefore provide information about the orbital energies of ligands on the surface of the QD.51-53 For example, delocalization of holes into phenyldithiocarbamate (PTC) ligands adsorbed onto the surfaces of CdS dots results in up to a 1 eV bathochromic shift of the first excitonic peak.54 A similar red shift, though weaker in magnitude, is generally observed upon adsorption of thiols.52,53 Adsorption of CN- ions to CdSe QDs localizes the electron in the QD core and induces a hypsochromic shift.55 Conversely, the spectral features of ligands sometimes reflect interaction with a QD surface. Recent studies have shown bathochromic shifts in the absorption spectra of cyanine dyes upon adsorption (and aggregation) on CdSe/ZnCdS QDs56 and CdSe QDs.57 The presence of cross-linking ligands on the surface of the QD can also be detected by spectral shifts in the excitonic peaks due to the electronic coupling between linked QDs. For example, the aggregation of two QDs bound to a single dithiol causes a red-shift in the band edge absorption peak of CdTe QDs.52,53 X-ray Photoelectron Spectroscopy (XPS). XPS measurements can differentiate between ligand atoms adsorbed to the QD and those co-deposited with the QDs in a thin film sample, and can often discern the binding motif(s) of the ligands from the unique oxidation states of the interfacial atoms.58-60 For example, Lobo et al.61 observed five different forms of sulfur and two forms of lead present in samples of PbS quantum dots. They attributed each of the five sulfur signatures to a different surface coordination environment. The sulfur species included (i) sulfur in the core of the crystal, (ii) sulfur within disulfide bonds, (iii) unpassivated sulfur atoms on the surface of the QD, (iv) sulfur passivated by TOP ligands, and (iv) sulfate, sulfite, and thiosulfate oxides. Similarly, the oxidation state of the Pb atoms on the surface of the QDis an indirect probe of the identity of the passivating ligands: the binding energy of the Pb2+ peak confirms the



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

Page 10 of 36

presence of a ligand with carboxylate headgroup. Similar work in our own lab demonstrated that XPS can be used to identify the presence of phosphorous- and nitrogen-containing groups in films of CdSe QDs subjected to various degrees of purification.3 We differentiated between surface-bound and free phosphorous-containing ligands through the binding energy of the P 2p peak. Energy-Dispersive X-ray Spectroscopy (EDS). Many Scanning, Transmission, and Scanning Transmission Electron Microscopes (SEM, TEM, and STEM, respectively) are now equipped with EDS systems for compositional analysis during imaging, and several groups have utilized it to confirm the presence of elements within the capping ligands of QDs.62-64 Depending upon the specific characteristics of the instrument used, the technique is limited to elements with atomic numbers greater than 11 (Sodium) or as low as 3 (Beryllium).65 The variable lower limit stems from whether the detector window is beryllium or a super-ultra-thin (SUTW) polymer.66,67 Even with the appropriate window, quantification of elements below Z = 11 require correction factors and trace analysis has a 1% detection limit. EDS is further limited in samples with high surface roughness as photons released from lower regions of the surface can be absorbed during impact with more elevated areas. The resulting error from this effect is amplified for elements with Z ≤ 10, with reports of errors up to 20%.68

Quantifying the Ligands Adsorbed to QDs Many applications of QDs for chemical detection, extraction of charge carriers, and fluorescence require quantification, in addition to identification, of ligands adsorbed to the surface of colloidal QDs. The primary challenges in counting ligands on the surface of the QD are finding a quantifiable analytical signal unique to the ligand of interest, and separating


Page 11 of 36

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


contributions to that signal from the free and bound ligands. The problem is further complicated by the inherent heterogeneity of ensembles of colloid QD-ligand complexes; there are many populations of QDs within the ensemble differentiated by the number of adsorbed ligands per QD. It is therefore necessary to use statistical distributions to accurately describe the surface coverage of ligands within a population of QD-ligand complexes. There are two groups of techniques to characterize the elemental composition of QD-ligand complexes: those that measure the sample in the solution phase, and thus probe the system at equilibrium, and those that require removal of the QD-ligand complexes from their native solutions. Table 1 lists a series of measured values for the number of ligands per unit area for various types and sizes of QDs, and the method used for the measurement. “Destructive” Elemental Analysis: ICP, XPS, and TGA. As a result of its high degree of analytical precision and sensitivity to many metallic elements, ICP(AES,MS) is an effective technique for elemental analysis of the inorganic cores of colloidal QDs.69 Applying this technique to analyze the ligand shell is difficult because many of the elements characteristic of the surfactants used to stabilize colloidal QDs (particularly O, N, P, and S) are poorly ionized or detected, and are present as impurities in the laboratory environment. Nonetheless, these methods can be used to quantify the surface coverage of some of the common QD-capping ligands. For example, we have used ICP-AES to determine that the alkylphosphonate layer on CdSe QDs synthesized with TOPO as the coordinating solvent contains a multilayer of Cd(phosphonate) complexes.5 Quantitative elemental analysis of the composition of QDs using XPS is more complicated than with ICP because of the sensitivity of the XPS measurement to the geometry of the sample. Signal of photoelectrons emitted by atoms closer to the surface within a film of QDs undergo



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

Page 12 of 36

smaller attenuation than atoms buried deeper within the sample, and are therefore overrepresented in the intensity of the acquired spectrum.3,56,70 Most studies using XPS to analyze the composition of nanostructured films employ a geometric model of the detection probability that weights the signals from each element by their probability of escaping the sample.3,71,72 Alternatively, one may calibrate the elemental analysis results from the XPS measurements to other quantitative methods such as ICP-AES.73 Most quantification studies by XPS concentrate on calculating elemental ratios between QD core elements and elements found within the binding ligands.74 Katari et al. used this ratio method to analyze the concentration of surface-bound ligands in films of CdSe QDs anchored to a gold substrate through alkane thiols.71 They found that the surface of the QD was between 30% and 60% covered by TOPO depending on the size of the QD. They ascribe the limited surface coverage to steric hindrance from the large cone-angle of TOPO, and proposed that the smaller dots had a greater surface coverage because the high radius of curvature of the surface of the QD provided more space for each TOPO ligand.75 Calculating elemental ratios of QD-ligand complexes also allows determination of the additional impurities that arise from processes such as atmospheric oxidation. Konstantatos et al. measured ratios of lead to sulfate impurities to calculate a PbS to PbSO4 ratio. The cited work found that three factors affected formation of oxidative impurities on the surface: exposure to air, washing with methanol, and annealing in air.76 Quantification of species such as surface oxides can be equally important to determining ligand concentrations as many impurities serve as trap states. There are limitations to using XPS to characterize ligands on QD surfaces. (i) Quantification by XPS may be complicated by overlapping peaks (for example, S and Se) for the elements of interest.77 (ii) Contamination of samples by adsorbed water, volatile organic carbons (VOCs),


Page 13 of 36

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


and other adventitious species make analysis of carbon and oxygen concentrations difficult.3,61 Quantitative analysis of the ligands using the O and C 1s peaks are, therefore, rare without further chemical alteration.78,79 (iii) Analysis using XPS requires the QDs be deposited as films on conducting substrates, and preparing these films from colloidal solutions may perturb the equilibrium between bound and free ligands, and change the concentration of ligands on the surface of the QD from that in the solution-phase.80,81 (iv) At low surface coverage (several ligands per QD), the detection limit of XPS may prevent quantitative analysis of the concentration of ligands despite the techniques capability of probing concentrations down to ~0.1% of the material.82,83 For instance, Beard et al. observed switching between n- and pcharacter in PbSe films upon hydrazine treatment, but were unable to detect the single adsorbate molecule per quantum dot hypothesized to produce the change in type of excess charge carriers.84 The total mass of ligands in samples of QD-ligand complexes can be determined by TGA through the different decomposition/vaporization temperatures of the ligands and the inorganic core.85,86 Within this experiment, one quantifies the mass of ligands by comparing the total mass loss associated with the ligand layer to the remaining mass, assigned to the inorganic core. If the identities of the ligands in the sample are known, and all of the ligands in the sample are adsorbed to the QD surface, the number of ligands per QD can be calculated directly from the mass ratio. This experiment has been used successfully toanalyze differences in the packing density of polymers on QD surfaces,66,81-83 to quantify ligand coverages on QDs upon synthesis and ligand exchange,87 and treatment for ligand removal.88,89 TGA can also be used to discriminate, through volatilization temperature, between ligands adsorbed to the surface of the QD and free ligands, physisorbed species, and loosely coordinated solvent molecules.90-92 This



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

Page 14 of 36

technique therefore provides thermodynamic information about the relative binding adsorption energies of different ligands by measuring the temperature necessary to dissociate bound ligands from the surface of the QD.92-94 Non-Destructive Elemental Analysis: NMR and Optical Methods.1H NMR is an accessible and affordable method for measuring the concentration of QD ligands present in the sample (Table 1).15,19,34,47 In order to successfully determine the concentration of bound ligands in the QD solution, it is necessary to (i) establish which NMR signals originate from protons of the ligands that are attached to the QD surface, and (ii) include an internal standard in the QD solution. The first requirement is straightforward when the sample is purified well enough such that it does not contain residual free ligands in solution, and/or when the chemical shift of at least one of the chemical groups in the surface-bound ligands is spectrally separated from those of the free ligands. The internal standard should be chosen such that it is chemically inert with respect to both QDs and the ligands and has proton resonances at different chemical shifts than the ligands of interest. Moreels et al., using a dibromomethane internal standard, successfully quantified the amounts of free and bound TOPO in a sample of InP QDs.34 We used ferrocene as an internal standard and resonances of vinyl protons in oleate and the protons in the methylene group alpha to sulfur in decanethiol to measure the concentrations of oleate and decanethiolate ligands bound to PbS QDs.19 We found treating oleate-capped QDs with decanethiol resulted in QDs coated with a mixture of decanethiolate (87%) and oleate (13%), and the QDs contained an overall average coverage of 150 ± 30 ligands per QD, Table 1.19 Concentrations of QDs on the order of 10-4 M were required to get adequate signal-to-noise. Such a high concentration may be inaccessible in some QD-ligand complexes due to the low solubility of the QDs or the low yield of the QD synthesis. It is important to note that, in order to achieve precise signal-integration


Page 15 of 36

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


values in cases when the low concentration of the QDs (and thus of ligands) requires multiple scans, it is necessary to allow the sample to completely relax before subsequent scan is initiated; in practice such a complete relaxation can be achieved by setting the relaxation delay to 5×T1.22 The strong changes in photoluminescence (PL) intensity upon adsorption of capping ligands combined with the high degree of sensitivity and precision of modern PL spectroscopy techniques make PL spectroscopy an attractive method to quantify the number of ligands adsorbed to QD surfaces.15,95,96 In order to determine the surface coverage of a ligand using PL measurements, however, one needs to understand the mechanism by which a ligand quenches or enhances the PL of the QD – more specifically, the functional form of the PL response of the QD to adsorption of ligands. Complicating the quantitative analysis is the dramatic sensitivity of the PL intensity of the QD to many environmental factors – such a local dielectric, temperature, QD aggregation, solvent impurities, oxidation, or stochastic trapping of excitonic charges on the QD surface –in addition to changes in surface coverage. Well-designed experiments will isolate the PL response of the QD to ligand adsorption from the PL response of the QD to competing environmental stimuli. Ligands can cause the PL intensity of QDs to increase by (i) passivating the surface of the QD by coordinating to trap states, (ii) removing quenching species from the surface of the QD or (iii) acting as energy transfer donors. Conversely, ligands can decrease the PL intensity of QDs by (i) forming trap states for the excitonic carriers, (ii) displacing passivating ligands or (iii) as energy or charge transfer acceptors.15 Determining the exact nature of the interactions between the ligand and the excited state of the QD can elucidate the functional dependence of the PL of the QDs to the adsorbed ligands. For example, we have found that ligands capable of accepting an electron (with time constants et< 100 ps) from photoexcitedCdS QDs quantitatively quench



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

Page 16 of 36

the PL of the QD upon adsorption of a single ligand.97 Comparing the timescale of electron transfer to the time scale of radiative recombination verifies the quantitative quenching from a single ligand adsorbed in a redox-active configuration. Single-QD fluorescence spectroscopy measurements of CdSe QD show that adsorption of a single octanethiol can decrease the PL QY by up to 50%.98 Once the relationship between the PL of the QD and the concentration of ligands on the surface has been established, one can fit plots of PL intensity vs. concentration of quenching (or enhancing) ligand as a convolution of the pre-determined PL response function and an appropriate binding isotherm. The Langmuir isotherm, due to its simplicity and broad applicability, is the most common model used to describe QD-ligand adsorption equilibria, though others,10,12,72,96,97,99,100 including the Freundlich and Fowler isotherms34,101 and the Stern Volmer102 model also apply in some cases. Another optical technique recently proven useful for quantifying the distribution of adsorbed ligands on QDs is transient absorption (TA).100 TA is a pump-probe spectroscopy generally performed on QD-ligand samples to monitor the dynamics of excitonic carriers (electrons and holes).103-106 In a typical experiment, a monochromatic pump pulse produces an excited state that is probed with a second pulse as a function of the time delay between the pump and probe. Transient absorption is useful for quantifying the number of ligands per QD if the ligand participates in charge transfer with the QD upon adsorption. This requirement is a major limitation of TA spectroscopy for analytical purposes, and severely restricts the number of samples for which it is appropriate. The most commonly used native ligands are not redox-active under typical experimental conditions, and only a subset of redox active ligands may adopt geometries on the surface of the QD that are suitable for charge transfer.


Page 17 of 36

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


The key to using TA spectroscopy to count surface ligands is that the observed rateof charge transfer to a redox-active ligand within an ensemble of QD-ligand complexes varies approximately linearly with the number of ligands bound (i.e., the number of available charge transfer pathways).100,107 One can therefore identify the average number of bound ligands per QD by measuring the charge transfer rate as a function of ligand concentration.108-110 In the regime of low concentration of the redox-active adsorbate, the yield of electron transfer is also proportional to the concentration of surface-bound ligands.108,111,112We have usedthe yield of electron transfer as measured by TA to determine the average and distribution of the number of viologen ligands adsorbed to the surfaces of CdSQDs,100 and the number of aminoferrocene ligands adsorbed to PbS QDs.19 Others have used it to quantify C60 ligands on core-shell QDs,113 and Rhodamine B ligands on CdSe QDs.114 The TA-based analytical technique specifically counts only ligands thatare in charge-transfer active geometries on the surface, and therefore may not yield the same values of binding constants obtained by NMR or other methods. This method of analysis is appropriate for samples too dilute to measure by NMR or those where the subset of redox active ligands, rather than the entire population of ligands, is the population of interest. In addition to the analytical challenge of measuring the concentration of bound and free ligands in colloidal solutions of QDs, developing quantitative models of QD-ligand adsorption processes is difficult because the detailed chemical structure of the QD-ligand interface is still poorly understood. Identifying the nature of a “binding site” on the surface of the QD and the number of potential binding sites per QD is of critical importance in analyzing surface coverage of ligands.96,102 For example, we have found that the number of available binding sites per QD



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

Page 18 of 36

depends sensitively on the absolute concentration of QDs,97 and the concentration and type of competing ligands present in the system.19

What is the structure (geometry) of the ligands on the surface of the QD? While the binding affinity for specific chemical head groups to various crystallographic faces of QDs can be estimated with electronic structure theory115,116 or inferred through measurement of competitive adsorption equilibria, the geometries of molecules within organic adlayers on small QDs, and the intermolecular structure of these adlayers, is a more complex problem. The suite of techniques for characterizing the intra- and intermolecular structure of ligands on the surface of QDs comprises those used for characterizing planar surfaces, and creative use of methods that probe the geometry, binding motif, and local environment of the ligands indirectly. Sum Frequency Generation. The structure of ligands on QD surfaces has been investigated using vibrational sum-frequency generation (SFG), a second-order nonlinear optical spectroscopy

with

submonolayer

sensitivity117-119

and

a

unique

selectivity

for

noncentrosymmetric environments such as a surface.120 The surface selectivity of SFG allows one to identify only the ligands attached directly to the QD without a background from free ligands in the bulk. Due to the noncentrosymmetric selectivity of SFG, signals from oscillating methylene stretches in long, well-ordered carbon chains cancel each other, while those at gauche defects add coherently. The relative magnitude of the signals arising from methylene stretches to those arising from methyl stretches is therefore a measure of ligand geometry and order on the QD surface.121-123 Previous SFG studies have detected decreasing order of straight-chain alkyl ligands with decreasing size of nanoparticles for both semiconductors105 and metals119,121 (Figure 4). Disorder in smaller QDs is due, in part, to the increased cone volume of available space for


Page 19 of 36

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


conformational reorganization of the ligand as the curvature of the surface increases, and, in part, to increasing disorder of the underlying semiconductor layer.105 For example, in CdSe QDs synthesized by the most common procedures, the tightest binding ligands are negatively charged alkylphosphonates, providing a driving force for Cd2+-enrichment of the surface. The form of this enrichment is a disordered Cd2+-phosphonate polymer, the thickness of which increases as the diameter of the QD decreases.105 Ligand disorder in these systems is therefore driven by both the geometry of the surface and its chemical composition. FT-IR. Fully trans-extended alkyl chains show lower frequency of the symmetric methylene C-H stretching mode than disordered chains containing gauche defects. FTIR spectroscopy is therefore, in some cases, a sensitive qualitative probe of alkyl chain conformation on the surface of QDs, as it is for planar surfaces. For example, Meulenberg et al. used this technique to show that the degree of crystallinity of the alkyl chains in hexadecylamine-coated CdSe QDs increases with the radius of the nanoparticles.48 NOESY and DOSY. As mentioned previously, NOESY establishes whether probed ligands are attached to the surface of a QD, even for systems in which the ligands are in a dynamic equilibrium between bound and free states.11,19,21,36,37 A combination of NOESY and DOSY can determine both the local environment of the ligand, and the rate of desorption of ligands from the QD surface.11,19,21,36,37 Such an approach has been used to show that alkylaminesundergo fast exchange on the surfaces of CdTe, ZnO,37 and CdSe11 QDswith kdesorption>50 s-1. We have used a similar approach to show that aminoferrocene undergoes fast exchange on the surface of PbS nanoparticles.19 Time-Resolved Absorption and Emission Measurements. Ligands that participate in charge transfer reactions are typically divided into three important segments: the anchor group that



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

Page 20 of 36

binds to the surface of the semiconductor, the electron donating moiety, and a bridge that connects the anchor to the donor. Because the rate of electron tunneling decreases exponentially with the distance between the donor and acceptor, trends in electron transfer rate as a function of linker length provide information about the orientation of the ligands on the surface. For example, Asbury et al.124,125 measured charge transfer rates using a system of Re dyes adsorbed to TiO2 nanocrystalline thin films through carboxylic acid-terminated alkyl chains of variable length. They observed that the electron transfer rate qualitatively agrees with nonadiabatic electron transfer theory except upon the first insertion of a methylene group, which they suggest moves the system from the adiabatic to the nonadiabatic regime. The otherwise exponential increase in charge transfer rate with increasing chain length suggests that the dye binds through the carboxylic acid and “stands up” on (i.e., is extended away from) the TiO2 surface. Similar results have been obtained for Re dyes anchored to SnO2 thin films and other systems.126,127 We have observed that the conformation of redox-active ligands on CdSe QDs is not as straightforward. We measured the rate of electron transfer from photoexcited CdSe QDs to viologen units within poly(viologen) co-deposited with the QDs into a thin film. The electron transfer rate in this system does not follow exponential distance dependence if one assumes that the native alkyl ligands separating the QD from the viologenare trans-extended shells; rather, we fit the dynamics using a collapsed ligand shell model (Figure 5).73

Conclusion The composition of ligands stabilizing semiconductor QDs frequently determines a number of QD properties that are crucial for their intended application. This review highlights the major analytical approaches that are most useful for characterizing this composition. While the cost of


Page 21 of 36

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


instrumentation for a few of the techniques we describe here (e.g., ultrafast spectroscopy) may be prohibitive for researchers who are considering working with semiconductor QDs, standard and well established analytical methods, such as NMR and photoluminescence, can be very effective in establishing the composition and structure of QD ligand shell. They are therefore a practical starting point for in-depth chemical characterization of these exciting nanomaterials.



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

Page 22 of 36

Tables and Figures

Table 1. Values for the number of ligands per unit area on the surface of a QD – for different ligands and types and sizes of QDs – as measured using the various techniques described in the the text. Quantum Dot

Radius (nm)

Ligand

Octylphosphonic Acid and Condensate PbSe 1.5-3.5 Oleic Acid CdSe 1.08 TOP(O,Se) CdSe 1.84 TOPO CdSe 0.93-3.02 TOPO PbS 1.7 Oleic Acid Decanethiol (87%) PbS 1.55 / Oleic Acid (13%) a Brackets indicate an average value. bLigands are “polymer”, rather than a monolayer, in this system. CdSe

1.8-3.4

Ligands/nm2

Technique

Reference

a,b

ICP-AES

Weiss128

a a .57 3.0 4.4 ± 0.8

1

H DOSY 1 H NMR TGA XPS 1 H NMR

Hens129 Murray15 Bawendi130 Alivisatos71 Weiss19

5.0 ± 0.8

1

Weiss19

H NMR

incorporated into a multi-layer Cd-phosphonate


Page 23 of 36

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


Figure 1. (a)1H NMR spectra of the pyridine treated CdSenanocrystals (OD = 10) after equilibrium with a given concentration of OA. (b)and(c) Peak fitting of the NMR spectra in the spectral range related to the α-hydrogen of OA. (d) NOESY spectrum of 4-bromoaniline (Br-An) and CdSe QDs in CD2Cl2 at [Br-An]:[QD] = 350:1 and of (e) Br-An in CD2Cl2 (no QDs) with the same [Br-An] (0.014 M) as in spectrum (d). (f) NOESY spectrum of 4-methoxyaniline (MeO-An) and CdSe QDs in CD2Cl2 at [MeO-An]:[QD] = 350:1, and of (g)MeO-An in CD2Cl2 (no QDs) with the same [MeO-An] (0.014 M) as in spectrum (f). The peaks between 6 and 8 ppm (figures d-g) are due to the protons on the aromatic rings of R-An (see insets for zoomed-in pictures of these regions). The peaks between 3 and 4 ppm (figures d-g) are due to amine protons, except for the peak at 3.8 ppm in the spectra of MeO-An, which is due to the methoxy protons. The peaks located between 2.5 and 0.5 ppm (figure d-g) correspond to methylene and methyl protons of the native OPA and PPA ligands. The colors in figures d-g indicate the relative phases and intensities of the signals.(a)-(c) Reprinted with permission from Ji, X.; Copenhaver, D.; Sichmeller, C.; Peng, X. J. Am. Chem. Soc.2008, 130, 5726.Copyright 2008 American Chemical Society. (d)-(f) Reprinted with permission from Donakowski, M. D.; Godbe, J. M.;



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

Page 24 of 36

Sknepnek, R.; Knowles, K. E.; Olvera de la Cruz, M.; Weiss, E. A. J. Phys. Chem. C.2010, 114, 22526.Copyright 2010 American Chemical Society.

Figure 2. 31P (1H) NMR spectra in CDCl3 at 25 °C of (A) a solution of CdSe nanoparticles synthesized using technical grade TOPO; (B) the solution after treatment of the nanoparticles in (A) with DMAP; and (C) the solution after treatment of the nanoparticles in (A) with propionate salt solution.Reprinted with permission fromKopping, J. T.; Patten, T. E. J. Am. Chem. Soc.2008, 130, 5689. 24 ACS Paragon Plus Environment

Page 25 of 36

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


Figure 3. (A) 1H NMR of a suspension of CdSe QD (545 μM, 2.9 nm) dissolved in d8THF after the addition of 46 and 221 mM oleic acid. Only the double bond region is shown. (B) DOSY of a suspension of CdSe QD (545 μM, 2.9 nm) dissolved in d8-THF after the addition of OA - 221 mM (OA:ODPA 5:1). (C) NOESY under the same conditions; only the negative NOE cross peaks are shown. Reprinted with permission from Gomes, R.; Hassinen, A.; Szczygiel, A.; Zhao, Q. A.; Vantomme, A.; Martins, J. C.; Hens, Z. J Phys Chem Lett 2011, 2, 145. Copyright American Chemical Society 2011.



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

Figure 4. (a) The full width at half maximum (FWHM, linewidth) of the Cd 3d5/2 peak in XPS is plotted as a function of QD radius for CdSe QDs synthesized with either 90% or 99% pure trioctylphosphine oxide (TOPO). Linewidths of the XPS peak reflect the heterogeneity of environments experienced by Cd2+ within a CdSe QD. The data suggest that the Cd2+ surface layer is more disordered in smaller QDs, and the environments become increasingly homogeneous as the size of the QD increases. (b) The ratio of the signal amplitudes of methyl and methylene symmetric vibrations observed using sum-frequency generation (SFG) spectroscopy plotted against the nanoparticles radius for CdSe QDs synthesized with either 90% or 99% pure TOPO. The signal ratio reveals the level of ligand disorder (see text), and suggests that as the size of the QD increases, the degree of ligand disorder decreases. (c,d) Schematic diagrams of relatively large (c) and small (d)CdSe QDs drawn according to the results presented in (a) and (b). Compared to the larger QDs, smaller QDs have a relatively disordered surface layer of Cd2+, and the native ligands contain more gauche defects due to the freedom allowed by an increased conical volume of available space. Reprinted with permission from Frederick, M. T.; Achtyl, J. L.; Knowles, K. E.; Weiss, E. A.; Geiger, F. M. J. Am. Chem. Soc.2011, 133, 7476.Copyright 2011 American Chemical Society. 26 ACS Paragon Plus Environment

Page 26 of 36

Page 27 of 36

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


Figure 5. (a) Variable length mercaptocarboxylic acid ligands act as a tunneling barrier against photoinduced electron transfer from CdSe QDs to polyviologen. (b) Marcus Theory predicts an exponential decrease in charge transfer rate with increasing separation distance between donor (QD) and acceptor (polyviologen). Fitting the ligand-length-dependent charge transfer rates to a model describing an electron tunneling through a rectangular barrier of the width expected for a trans-extended ligand shell is shown in red dashes. The weaker dependence of the photoinduced electron transfer rate on linker length, n, suggests a collapsed shell is more appropriate. A best fit is obtained for a condensed ligand shell with 5.6 carbon chains per square nanometer.Reprinted with permission from Tagliazucchi, M.; Tice, D. B.; Sweeney, C. M.; Morris-Cohen, A. J.; Weiss, E. A. ACS Nano.2011, 5, 9907.Copyright 2011 American Chemical Society.



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

Page 28 of 36

References. (1)

Petkov, V.; Moreels, I.; Hens, Z.; Ren, Y. Phys. Rev. B 2010, 81, 241304/1.

(2)

Kopping, J. T.; Patten, T. E. J. Am. Chem. Soc. 2008, 130, 5689.

(3)

Morris-Cohen, A. J.; Donakowski, M. D.; Knowles, K. E.; Weiss, E. A. J. Phys.

Chem. C 2010, 114, 897. (4)

Owen, J. S.; Park, J.; Trudeau, P.-E.; Alivisatos, A. P. J. Am. Chem. Soc. 2008,

130, 12279. (5)

Morris-Cohen, A. J.; Frederick, M. T.; Lilly, G. D.; McArthur, E. A.; Weiss, E. A.

J. Phys. Chem. Lett. 2010, 1, 1078. (6)

Wang, F.; Tang, R.; Buhro, W. E. Nano Lett. 2008, 8, 3521.

(7)

Aldana, J.; Wang, Y. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 8844.

(8)

Majetich, S. A.; Carter, A. C.; Belot, J.; McCullough, R. D. J. Phys. Chem. 1994,

98, 13705. (9)

Aldana, J.; Lavelle, N.; Wang, Y.; Peng, X. J. Am. Chem. Soc. 2005, 127, 2496.

(10)

Ji, X.; Copenhaver, D.; Sichmeller, C.; Peng, X. J. Am. Chem. Soc. 2008, 130,

(11)

Hassinen, A. Moreels, I. de Mello Doneg , C. Martins, . C. Hens, Z. J. Phys.

5726.

Chem. Lett. 2010, 1, 2577. (12)

Donakowski, M. D.; Godbe, J. M.; Sknepnek, R.; Knowles, K. E.; Olvera de la

Cruz, M.; Weiss, E. A. J. Phys. Chem. C 2010, 114, 22526. (13)

Gomes, R.; Hassinen, A.; Szczygiel, A.; Zhao, Q.; Vantomme, A.; Martins, J. C.;

Hens, Z. J. Phys. Chem. Lett. 2011, 2, 145. (14)

Milliron, D. J.; Alivisatos, A. P.; Pitois, C.; Edder, C.; Fréchet, J. M. J. Adv.

Mater. 2003, 15, 58. (15)

Kalyuzhny, G.; Murray, R. W. J. Phys. Chem. B 2005, 109, 7012.

(16)

Schmelz, O.; Mews, A.; Basche, T.; Herrmann, A.; Mullen, K. Langmuir 2001,

17, 2861. (17)

Kim, H.; Suh, M.; Kwon, B.-H.; Jang, D. S.; Kim, S. W.; Jeon, D. Y. J. Colloid

Interface Sci. 2011, 363, 703. (18)

Dorokhin, D.; Tomczak, N.; Velders, A. H.; Reinhoudt, D. N.; Vancso, G. J. J.

Phys. Chem. C 2009, 113, 18676. 28 ACS Paragon Plus Environment

Page 29 of 36

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


(19)

Malicki, M.; Knowles, K. E.; Weiss, E. A. Chem. Comm. 2012.

(20)

Chen, C. T.; Munot, Y. S.; Salunke, S. B.; Wang, Y. C.; Lin, R. K.; Lin, C. C.;

Chen, C. C.; Liu, Y. H. Adv. Funct. Mater. 2008, 18, 527. (21)

Coppel, Y.; Spataro, G.; Pagès, C.; Chaudret, B.; Maisonnat, A.; Kahn, M. L.

Chem.--Eur. J. 2012, 18, 5384. (22)

Bakhmutov, V. I. Practical NMR relaxation for chemists; JohnWiley & Sons,

Ltd: n.p., 2004. (23)

Malicki, M.; Hales, J. M.; Rumi, M.; Barlow, S.; McClary, L.; Marder, S. R.;

Perry, J. W. Phys. Chem. Chem. Phys. 2010, 12, 6267. (24)

Sachleben, J. R.; Colvin, V.; Emsley, L.; Wooten, E. W.; Alivisatos, A. P. J.

Phys. Chem. B 1998, 102, 10117. (25)

Sachleben, J. R.; Wooten, E. W.; Emsley, L.; Pines, A.; Colvin, V. L.; Alivisatos,

A. P. Chem. Phys. Lett. 1992, 198, 431. (26)

Shen, L.; Soong, R.; Wang, M. F.; Lee, A.; Wu, C.; Scholes, G. D.; Macdonald,

P. M.; Winnik, M. A. J. Phys. Chem. B 2008, 112, 1626. (27)

Evans, C. M.; Evans, M. E.; Krauss, T. D. J. Am. Chem. Soc. 2010, 132, 10973.

(28)

Wang, F.; Tang, R.; Buhro, W. E. Nano Lett. 2008, 8, 3521.

(29)

Anderson, N. C.; Owen, J. S. Submitted

(30)

Wang, F.; Tang, R.; Kao, J. L. F.; Dingman, S. D.; Buhro, W. E. J. Am. Chem.

Soc. 2009, 131, 4983. (31)

Becerra, L. R.; Murray, C. B.; Griffin, R. G.; Bawendi, M. G. J. Chem. Phys.

1994, 100, 3297. (32)

Berrettini, M. G.; Braun, G.; Hu, J. G.; Strouse, G. F. J. Am. Chem. Soc. 2004,

126, 7063. (33)

Price, W. S. NMR studies of translational motion; Cambridge University Press:

Cambridge, 2009. (34)

Moreels, I.; Martins, J. C.; Hens, Z. Sens. Actuators, B 2007, 126, 283.

(35)

Hens, Z.; Moreels, I.; Martins, J. C. ChemPhysChem 2005, 6, 2578.

(36)

Coppel, Y.; Spataro, G.; Colliere, V.; Chaudret, B.; Mingotaud, C.; Maisonnat,

A.; Kahn, M. L. Eur. J. Inorg. Chem. 2012, 2691.



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

(37)

Page 30 of 36

Fritzinger, B.; Martins, J. C.; Moreels, I.; Lommens, P.; Koole, R.; Hens, Z. J.

Am. Chem. Soc. 2009, 131, 3024. (38)

Canzi, G.; Kubiak, C. P.; Mrse, A. A. J. Phys. Chem. C 2011, 115, 7972.

(39)

Clore, G. M.; Gronenborn, A. M. J. Magn. Reson. 1982, 48, 402.

(40)

Guo, Y.; Moffitt, M. G. Macromolecules 2007, 40, 5868.

(41)

Zhang, K. X.; Yu, Y. X.; Sun, S. Q. Appl. Surf. Sci. 2012, 258, 7658.

(42)

Yu, Y. X.; Zhang, K. X.; Sun, S. Q. Appl. Surf. Sci. 2012, 258, 7181.

(43)

Cooper, J. K.; Franco, A. M.; Gul, S.; Corrado, C.; Zhang, J. Z. Langmuir 2011,

27, 8486. (44)

Abd El-sadek, M. S.; Nooralden, A. Y.; Babu, S. M.; Palanisamy, P. K. Opt.

Commun. 2011, 284, 2900. (45)

Chen, S. T.; Zhang, X. L.; Zhang, Q. H.; Hou, X. M.; Zhou, Q.; Yan, J. L.; Tan,

W. H. J. Lumin. 2011, 131, 947. (46)

Bailon-Ruiz, S.; Alamo-Nole, L.; Perales-Perez, O. Current Nanoscience 2012, 8,

(47)

Hua, F.; Swihart, M. T.; Ruckenstein, E. Langmuir 2005, 21, 6054.

(48)

Meulenberg, R. W.; Strouse, G. F. J. Phys. Chem. B 2001, 105, 7438.

(49)

Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854.

(50)

Jasieniak, J.; Smith, L.; Embden, J. v.; Mulvaney, P.; Califano, M. J. Phys. Chem.

202.

C 2009, 113, 19468. (51)

Frederick, M. T.; Weiss, E. A. ACS Nano 2010, 4, 3195.

(52)

Koole, R.; Luigjes, B.; Tachiya, M.; Pool, R.; Vlugt, T. J. H.; de Mello Donega,

C.; Meijerink, A.; Vanmaekelbergh, D. The Journal of Physical Chemistry C 2007, 111, 11208. (53)

Koole, R.; Liljeroth, P.; de Mello Donegá, C.; Vanmaekelbergh, D.; Meijerink, A.

J. Am. Chem. Soc. 2006, 128, 10436. (54)

Frederick, M. T.; Amin, V. A.; Cass, L. C.; Weiss, E. A. Nano Lett. 2011, 11,

(55)

Sarkar, S. K.; Chandrasekharan, N.; Gorer, S.; Hodes, G. Appl. Phys. Lett. 2002,

5455.

81, 5045. (56)

Walker, B. J.; Nair, G. P. Marshall, . . Bulovi ,

Chem. Soc. 2009, 131, 9624. 30 ACS Paragon Plus Environment

. Bawendi, M.

. J. Am.

Page 31 of 36

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


(57)

McArthur, E. A.; Godbe, J. M.; Tice, D. B.; Weiss, E. A. J. Phys. Chem. C 2012,

116, 6136. (58)

Shallcross, R. C.; D'Ambruoso, G. D.; Korth, B. D.; Hall, H. K.; Zheng, Z.; Pyun,

J.; Armstrong, N. R. J. Am. Chem. Soc. 2007, 129, 11310. (59)

Cheng, L.; Liu, X.; Lei, J.; Ju, H. Anal. Chem. 2010, 82, 3359.

(60)

Zhang, T.; Zhao, H.; Riabinina, D.; Chaker, M.; Ma, D. J. Phys. Chem. C 2010,

114, 10153. (61)

Lobo, A.; Möller, T.; Nagel, M.; Borchert, H.; Hickey, S. G.; Weller, H. J. Phys.

Chem. B 2005, 109, 17422. (62)

Ke, Y.; Kailasa, S. K.; Wu, H.-F.; Chen, Z.-Y. Talanta 2010, 83, 178.

(63)

Erogbogbo, F.; Yong, K.-T.; Roy, I.; Xu, G.; Prasad, P. N.; Swihart, M. T. ACS

Nano 2008, 2, 873. (64)

Yan, C.; Tang, F.; Li, L.; Li, H.; Huang, X.; Chen, D.; Meng, X.; Ren, J.

Nanoscale Res. Lett. 2010, 5, 189. (65)

Storey, R. A.; Ymen, I.; Ymén, I. Solid State Characterization of

Pharmaceuticals; John Wiley & Sons, 2011. (66)

Mondi, C.; Leifer, K.; Mavrocordatos, D.; Perret, D. J. Microsc. 2002, 207, 180.

(67)

Kowbel, W.; Wapner, P. G.; Wright, M. A. J. Phys. Chem. Solids 1988, 49, 1279.

(68)

Stadelmann, P.; Leifer, K.; Verdon, C. Ultramicroscopy 1995, 58, 35.

(69)

Montoro Bustos, A. R.; Encinar, J. R.; Fernandez-Argueelles, M. T.; Costa-

Fernandez, J. M.; Sanz-Medel, A. Chem. Commun. (Cambridge, U. K.) 2009, 3107. (70)

Slavin, J. W. J.; Jarori, U.; Zemlyanov, D.; Ivanisevic, A. J. Electron Spectrosc.

Relat. Phenom. 2009, 172, 47. (71)

Katari, J. E. B.; Colvin, V. L.; Alivisatos, A. P. J. Phys. Chem. 1994, 98, 4109.

(72)

Bullen, C.; Mulvaney, P. Langmuir 2006, 22, 3007.

(73)

Tagliazucchi, M.; Tice, D. B.; Sweeney, C. M.; Morris-Cohen, A. J.; Weiss, E. A.

ACS Nano 2011, 5, 9907. (74)

Zhang, H.; Yang, B. Thin Solid Films 2002, 418, 169.

(75)

Deng, Z.; Lie, F. L.; Shen, S.; Ghosh, I.; Mansuripur, M.; Muscat, A. J. Langmuir

2008, 25, 434.



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

(76)

Page 32 of 36

Konstantatos, G.; Howard, I.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem, E.;

Levina, L.; Sargent, E. H. Nature 2006, 442, 180. (77)

Zorn, G.; Dave, S. R.; Gao, X.; Castner, D. G. Anal. Chem. 2011, 83, 866.

(78)

Desimoni, E.; Casella, G. I.; Morone, A.; Salvi, A. M. Surf. Interface Anal. 1990,

15, 627. (79)

Vander Wal, R. L.; Bryg, V. M.; Hays, M. D. Anal. Chem. 2011, 83, 1924.

(80)

Vallée, A.; Humblot, V.; Méthivier, C.; Pradier, C.-M. Surf. Sci. 2008, 602, 2256.

(81)

Karakoti, A. S.; Sanghavi, S.; Nachimuthu, P.; Yang, P.; Thevuthasan, S. J. Phys.

Chem. Lett. 2011, 2, 2925. (82)

Tengstedt, C.; de Jong, M. P.; Kanciurzewska, A.; Carlegrim, E.; Fahlman, M.

Phys. Rev. Lett. 2006, 96, 057209. (83)

Chan, Y.-H.; Chen, J.; Liu, Q.; Wark, S. E.; Son, D. H.; Batteas, J. D. Anal.

Chem. 2010, 82, 3671. (84)

Beard, M. C.; Midgett, A. G.; Law, M.; Semonin, O. E.; Ellingson, R. J.; Nozik,

A. J. Nano Lett. 2009, 9, 836. (85)

Porter, V. J.; Geyer, S.; Halpert, J. E.; Kastner, M. A.; Bawendi, M. G. J. Phys.

Chem. C 2008, 112, 2308. (86)

Chen, H. S.; Wang, S. J. J.; Lo, C. J.; Chi, J. Y. Appl. Phys. Lett. 2005, 86,

131905. (87)

Yu, W. W.; Chang, E.; Falkner, J. C.; Zhang, J.; Al-Somali, A. M.; Sayes, C. M.;

Johns, J.; Drezek, R.; Colvin, V. L. J. Am. Chem. Soc. 2007, 129, 2871. (88)

Dong, A.; Ye, X.; Chen, J.; Kang, Y.; Gordon, T.; Kikkawa, J. M.; Murray, C. B.

J. Am. Chem. Soc. 2010, 133, 998. (89)

Leatherdale, C. A.; Woo, W. K.; Mikulec, F. V.; Bawendi, M. G. J. Phys. Chem.

B 2002, 106, 7619. (90)

Bagaria, H. G.; Kini, G. C.; Wong, M. S. J. Phys. Chem. C 2010, 114, 19901.

(91)

Neelgund, G. M.; Oki, A. Applied Catalysis B: Environmental 2011, 110, 99.

(92)

Niu, Y. H.; Munro, A. M.; Cheng, Y. J.; Tian, Y. Q.; Liu, M. S.; Zhao, J. L.;

Bardecker, J. A.; Jen- a Plante, I.

inger, D. S. en, A. K. Y. Adv. Mater. 2007, 19, 3371.


Page 33 of 36

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


(93) Konstantatos,

Pattantyus-Abraham, A. . Debnath, R.

. Kramer, I.

. Barkhouse, A. R.

evina, . Raabe, I. Nazeeruddin, M. K.

ang,

.

r tzel, M. Sargent,

E. H. ACS Nano 2010, 4, 3374. (94)

Webber, D. H.; Brutchey, R. L. J. Am. Chem. Soc. 2011, 134, 1085.

(95)

Knowles, K. E.; Tice, D. B.; McArthur, E. A.; Solomon, G. C.; Weiss, E. A. J.

Am. Chem. Soc. 2009, 132, 1041. (96)

Ginger, D. S.; Munro, A. M.; Jen-La Plante, I.; Ng, M. S. J. Phys. Chem. C 2007,

111, 6220. (97)

Morris-Cohen, A. J.; Vasilenko, V.; Amin, V. A.; Reuter, M. G.; Weiss, E. A.

ACS Nano 2011. (98)

Ginger, D. S.; Munro, A. M. Nano Lett. 2008, 8, 2585.

(99)

Koole, R.; Schapotschnikow, P.; Donega, C. D.; Vlugt, T. J. H.; Meijerink, A.

ACS Nano 2008, 2, 1703. (100) Morris-Cohen, A. J.; Frederick, M. T.; Cass, L. C.; Weiss, E. A. J. Am. Chem. Soc. 2011, 133, 10146. (101) Moreels, I.; Martins, J. C.; Hens, Z. ChemPhysChem 2006, 7, 1028. (102) Boldt, K.; Jander, S.; Hoppe, K.; Weller, H. ACS Nano 2011. (103) Klimov, V. I. J. Phys. Chem. B 2000, 104, 6112. (104) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385. (105) Frederick, M. T.; Achtyl, J. L.; Knowles, K. E.; Weiss, E. A.; Geiger, F. M. J. Am. Chem. Soc. 2011, 133, 7476. (106) McArthur, E. A.; Morris-Cohen, A. J.; Knowles, K. E.; Weiss, E. A. J. Phys. Chem. B 2010, 114, 14514. (107) Boulesbaa, A.; Issac, A.; Stockwell, D.; Huang, Z.; Huang, J.; Guo, J.; Lian, T. J. Am. Chem. Soc. 2007, 129, 15132. (108) Huang, J. E.; Huang, Z. Q.; Jin, S. Y.; Lian, T. Q. J. Phys. Chem. C 2008, 112, 19734. (109) Cooney, R. R.; Sewall, S. L.; Dias, E. A.; Sagar, D. M.; Anderson, K. E. H.; Kambhampati, P. Phys. Rev. B 2007, 75.



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

Page 34 of 36

(110) Klimov, V. I.; Schwarz, C. J.; McBranch, D. W.; Leatherdale, C. A.; Bawendi, M. G. Phys. Rev. B 1999, 60, R2177. (111) Sadhu, S.; Tachiya, M.; Patra, A. J. Phys. Chem. C 2009, 113, 19488. (112) Tachiya, M. J. Chem. Phys. 1982, 76, 340. (113) Song, N.; Zhu, H.; Jin, S.; Zhan, W.; Lian, T. ACS Nano 2010, 5, 613. (114) Boulesbaa, A.; Huang, Z.; Wu, D.; Lian, T. J. Phys. Chem. C 2009, 114, 962. (115) Puzder, A.; Williamson, A. J.; Zaitseva, N.; Galli, G.; Manna, L.; Alivisatos, A. P. Nano Lett. 2004, 4, 2361. (116) Rempel, J. Y.; Trout, B. L.; Bawendi, M. G.; Jensen, K. F. J. Phys. Chem. B 2006, 110, 18007. (117) Kristalyn, C. B.; Watt, S.; Spanninga, S. A.; Barnard, R. A.; Nguyen, K.; Chen, Z. J. Colloid Interface Sci. 2011, 353, 322. (118) Miranda, P. B.; Xu, L.; Shen, Y. R.; Salmeron, M. Phys. Rev. Lett. 1998, 81, 5876. (119) Bordenyuk, A. N.; Weeraman, C.; Yatawara, A.; Jayathilake, H. D.; Stiopkin, I.; Liu, Y.; Benderskii, A. V. J. Phys. Chem. C 2007, 111, 8925. (120) Boyd, R. W. Nonlinear Optics; 2 ed.; Academic Press: San Diego, 2003. (121) Weeraman, C.; Yatawara, A. K.; Bordenyuk, A. N.; Benderskii, A. V. J. Am. Chem. Soc. 2006, 128, 14244. (122) Messmer, M. C.; Conboy, J. C.; Richmond, G. L. J. Am. Chem. Soc. 1995, 117, 8039. (123) Holman, J.; Ye, S.; Neivandt, D. J.; Davies, P. B. J. Am. Chem. Soc. 2004, 126, 14322. (124) Asbury, J. B.; Hao, E.; Wang, Y.; Lian, T. J. Phys. Chem. B 2000, 104, 11957. (125) Asbury, J. B.; Hao, E.; Wang, Y.; Ghosh, H. N.; Lian, T. J. Phys. Chem. B 2001, 105, 4545. (126) Galoppini, E. Coord. Chem. Rev. 2004, 248, 1283. (127) Anderson, N. A.; Ai, X.; Chen, D.; Mohler, D. L.; Lian, T. J. Phys. Chem. B 2003, 107, 14231. (128) Morris-Cohen, A. J.; Frederick, M. T.; Lilly, G. D.; McArthur, E. A.; Weiss, E. A. J Phys Chem Lett 2010, 1, 1078. 34 ACS Paragon Plus Environment

Page 35 of 36

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


(129) Moreels, I.; Fritzinger, B.; Martins, J. C.; Hens, Z. J. Am. Chem. Soc. 2008, 130, 15081. (130) Kuno, M.; Lee, J. K.; Dabbousi, B. O.; Mikulec, F. V.; Bawendi, M. G. J. Chem. Phys. 1997, 106, 9869.



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

TOC figure


Page 36 of 36

Chemical, Structural, and Quantitative Analysis of the Ligand Shells of

Recommend Documents