Surface Chemistry Of Nanocrystals Research Articles

A mineralogical view of apatitic biomaterials

Biomaterials are synthetic compounds and composites that replace or assist missing or damaged tissue or organs. This review paper addresses Ca phosphate biomaterials that are used as aids to or substitutes for bones and teeth. The viewpoint taken is that of mineralogists and geochemists interested in (carbonated) hydroxylapatite, its range of compositions, the conditions under which it can be synthesized, and how it is used as a biomaterial either alone or in a composite. Somewhat counter-intuitively, the goal of most medical or materials science researchers in this field is to emulate the properties of bone and tooth, rather than the hierarchically complex materials themselves. The absence of a directive to mimic biological reality has permitted the development of a remarkable range of approaches to apatite synthesis and post-synthesis processing. Multiple means of synthesis are described from low-temperature aqueous precipitation, sol-gel processes, and mechanosynthesis to high-temperature solid-state reactions and sintering up to 1000 °C. The application of multiple analytical techniques to characterize these apatitic, frequently nanocrystalline materials is discussed. An online supplement 1 details the specific physical and chemical forms in which synthetic apatite and related Ca phosphate phases are used in biomaterials. The implications from this overview are the enhanced recognition of the structurally and chemically accommodating nature of the apatite phase, insight into the effects of synthesis techniques on the specific properties of minerals (specifically apatite), and the importance of surface chemistry of apatite nanocrystals. The wide range of synthesis techniques, types of analytical characterization, and applications to human health associated with apatite are a non-geological demonstration of the power of mineralogy.

Jonathan De Roo.

"My first experiment was an absolute failure. The glassware broke and a thermometer was destroyed. The best advice I have ever been given is to stop bothering about things you cannot influence." This and more about Jonathan De Roo can be found on page 1137.

1D nanocrystals with precisely controlled dimensions, compositions, and architectures.

The ability to synthesize a diverse spectrum of one-dimensional (1D) nanocrystals presents an enticing prospect for exploring nanoscale size- and shape-dependent properties. Here we report a general strategy to craft a variety of plain nanorods, core-shell nanorods, and nanotubes with precisely controlled dimensions and compositions by capitalizing on functional bottlebrush-like block copolymers with well-defined structures and narrow molecular weight distributions as nanoreactors. These cylindrical unimolecular nanoreactors enable a high degree of control over the size, shape, architecture, surface chemistry, and properties of 1D nanocrystals. We demonstrate the synthesis of metallic, ferroelectric, upconversion, semiconducting, and thermoelectric 1D nanocrystals, among others, as well as combinations thereof.

(Invited) Adventures in Si Nanocrystal Surface Chemistry....Controlling Optical Properties and so Much More

Silicon nanocrystals (SiNCs) have been attracting attention as active materials in a variety of proto-type devices including, solar cells, light-emitting diodes, and photodetectors. These, and other device structures require well-defined materials with predictable properties. Traditionally SiNC surfaces are rendered processable and stable toward oxidation by employing a variations of the general hydrosilylation reaction; they all involve the addition of a silicon-hydride bond in the SiNC surface across a carbon-carbon double (or triple) bond and affords a “monolayer” attached through a robust silicon-carbon linkage. Recently, it has come to light that typical hydrosilylation conditions can afford insulating multilayers that can render SiNCs electrically inactive. This finding led us to explore alternative functionalization protocols. In doing so, we discovered that surface chemistry provides another degree of freedom with which SiNC properties may be tailored. For example, it can be used to tailor the photoluminescent response throughout the visible spectrum, used to prepare SiNC/polymer hybrids, and even induce reactivity that provides polymerization of workhorse electronic polymers without the use of expensive transition metal catalysts. This presentation will include a discussion of our recent exploration into SiNC surface chemistry, new reactive platforms that have opened the door to new functional materials, and what our discoveries mean for the future of these fascinating materials.

Oxide Nanocrystal Model Catalysts.

Model catalysts with uniform and well-defined surface structures have been extensively employed to explore structure-property relationships of powder catalysts. Traditional oxide model catalysts are based on oxide single crystals and single crystal thin films, and the surface chemistry and catalysis are studied under ultrahigh-vacuum conditions. However, the acquired fundamental understandings often suffer from the "materials gap" and "pressure gap" when they are extended to the real world of powder catalysts working at atmospheric or higher pressures. Recent advances in colloidal synthesis have realized controlled synthesis of catalytic oxide nanocrystals with uniform and well-defined morphologies. These oxide nanocrystals consist of a novel type of oxide model catalyst whose surface chemistry and catalysis can be studied under the same conditions as working oxide catalysts. In this Account, the emerging concept of oxide nanocrystal model catalysts is demonstrated using our investigations of surface chemistry and catalysis of uniform and well-defined cuprous oxide nanocrystals and ceria nanocrystals. Cu2O cubes enclosed with the {100} crystal planes, Cu2O octahedra enclosed with the {111} crystal planes, and Cu2O rhombic dodecahedra enclosed with the {110} crystal planes exhibit distinct morphology-dependent surface reactivities and catalytic properties that can be well correlated with the surface compositions and structures of exposed crystal planes. Among these types of Cu2O nanocrystals, the octahedra are most reactive and catalytically active due to the presence of coordination-unsaturated (1-fold-coordinated) Cu on the exposed {111} crystal planes. The crystal-plane-controlled surface restructuring and catalytic activity of Cu2O nanocrystals were observed in CO oxidation with excess oxygen. In the propylene oxidation reaction with O2, 1-fold-coordinated Cu on Cu2O(111), 3-fold-coordinated O on Cu2O(110), and 2-fold-coordinated O on Cu2O(100) were identified as the active sites, respectively, to produce acrolein, propylene oxide, and CO2. Ceria rods enclosed with the {110} and {100} crystal planes, ceria cubes enclosed with the {100} crystal planes, and ceria octahedra enclosed with the {111} crystal planes exhibit distinct morphology-dependent oxygen vacancy concentrations and structures that can be well correlated with the surface compositions and structures of exposed crystal planes. Consequently, the metal-ceria interactions, structures, and catalytic performances of ceria-supported catalysts depend on the CeO2 morphology. Our results comprehensively reveal the morphology-dependent surface chemistry and catalysis of oxide nanocrystals that not only greatly deepen the fundamental understanding of oxide catalysis but also demonstrate a morphology-engineering strategy to optimize the catalytic performance of oxide catalysts. These results adequately exemplify the concept of oxide nanocrystal model catalysts for the fundamental investigations of oxide catalysis without the "materials gap" and "pressure gap". With the structure-catalytic property relationships learned from oxide nanocrystal model catalyst studies and the advancement of controlled-synthesis methods, it is promising to realize the structural design and controlled synthesis of novel efficient oxide catalysts in the future.

All-Inorganic Germanium Nanocrystal Films by Cationic Ligand Exchange.

We introduce a new paradigm for group IV nanocrystal surface chemistry based on room temperature surface activation that enables ionic ligand exchange. Germanium nanocrystals synthesized in a gas-phase plasma reactor are functionalized with labile, cationic alkylammonium ligands rather than with traditional covalently bound groups. We employ Fourier transform infrared and (1)H nuclear magnetic resonance spectroscopies to demonstrate the alkylammonium ligands are freely exchanged on the germanium nanocrystal surface with a variety of cationic ligands, including short inorganic ligands such as ammonium and alkali metal cations. This ionic ligand exchange chemistry is used to demonstrate enhanced transport in germanium nanocrystal films following ligand exchange as well as the first photovoltaic device based on an all-inorganic germanium nanocrystal absorber layer cast from solution. This new ligand chemistry should accelerate progress in utilizing germanium and other group IV nanocrystals for optoelectronic applications.

Controlling Solid-Gas Reactions at Nanoscale for Enhanced Thin Film Morphologies and Device Performances in Solution-Processed Cu2ZnSn(S,Se)4 Solar Cells.

Using Cu2ZnSn(S,Se)4 (CZTSSe) as a model system, we demonstrate the kinetic control of solid-gas reactions at nanoscale by manipulating the surface chemistry of both sol-gel nanoparticles (NPs) and colloidal nanocrystals (NCs). Specifically, we first identify that thiourea (commonly used as sulfur source in sol-gel processes for metal sulfides) can transform into melamine upon film formation, which serves as surface ligands for as-formed Cu2ZnSnS4 (CZTS) NPs. We further reveal that the presence of these surface ligands can significantly affect the outcome of the solid-gas reactions, which enables us to effectively control the selenization process during the fabrication of CZTSSe solar cells and achieve optimal film morphologies (continuous large grains) by fine-tuning the amount of surface ligands used. Such enhancement leads to better light absorption and allows us to achieve 6.5% efficiency from CZTSSe solar cells processed via a sol-gel process using nontoxic, low boiling point mixed solvents. We believe our discovery that the ligand of particulate precursors can significantly affect solid-gas reactions is universal to solid-state chemistry and will boost further research in both understanding the fundamentals of solid-state reactions at nanoscale and taking advantage of these reactions to fabricate crystalline thin film semiconductors with better morphologies and performances.

(Invited) Colloidal Nanocrystal Composites with High and Low Thermal Conductivity

We present our work on thermal transport in colloidal nanocrystal composites within the context of thermoelectrics and thermal energy management. Colloidal nanocrystals consist of an inorganic crystalline core with short ligand molecules bound to the surface. These nanocrystals can be made with excellent control over size, shape, and composition, thereby enabling microstructural control over thermal energy transport. Our first two topics focus on low thermal conductivity materials for thermoelectricity: semiconductor nanocrystal thin films and semiconductor matrix – semiconductor nanoparticle composites. Our last topic focuses on metal matrix – metal nanoparticle composites with high thermal conductivity, which can be used to thermally manage transient power spikes in high power electronics. For the first topic, we systematically study the effect of nanocrystal diameter and surface chemistry on thermal transport in lead chalcogenide nanocrystal thin films. These samples exhibit very low thermal conductivities ranging from 0.1 to 0.5 W/m-K and we find that larger nanocrystals and shorter ligands generally lead to higher thermal conductivities. Our measurements also show that the effect of surface chemistry on thermal transport becomes more pronounced as nanocrystal diameter decreases. For the second topic, we focus on polycrystalline In2Se3 matrices with embedded CdSe nanocrystals ranging from 0 to ~100 vol%. We find that the thermal conductivities of these composites are very low over the entire nanocrystal volume fraction range and vary from 0.3 to 0.5 W/m-K. We also find that the presence of CdSe nanocrystals strongly effects the formation of the In2Se3 matrix (i.e. grain orientation and size as well as ternary phase formation). Lastly, we present our work on phase change Bi nanocrystals embedded in a solid Ag-matrix. The latent heat of the phase change Bi nanocrystals can absorb large quantities of transient heat generated during power spikes in power electronics operation. In addition, fast thermal energy transport is facilitated by the Ag-matrix, which leads to high composite thermal conductivities ranging from 101 to 102 W/m-K. We also use size-dependent melting of the phase change nanocrystals as a design variable for the thermal energy absorption temperature.

Flow through Fluorescence Detection of Phosphate in Human Saliva Based on Sensitized Turn-On Photoluminescence of CdS Quantum Dots

ABSTRACTThe development of fluorescent turn-on probes for the determination of inorganic anions based on fluorescent semiconductor nanocrystals (quantum dots) has recently attracted great attention. Methods reported so far enable anion sensing at elevated concentration levels and usually with low selectivity against other abundant matrix components. In this work, we describe an alternative approach that combines the appropriate modulation of nanocrystal surface chemistry (i.e., stabilization) with rational design and modulation of the experimental conditions (i.e., flow injection analysis) in order to deliver an expedient flow through method that enables the determination of phosphate at low concentration levels and with good selectivity. The developed method was successfully applied to the determination of phosphate in artificial and human saliva with good analytical features in terms of reproducibility and repeatability (2.3–9.2%), recoveries from fortified samples (96.0–112.0%), linearity of response curves to phosphate concentrations (R2 > 0.99), and satisfactory sensitivity (<1 µM), providing a significant improvement to previous fluorescent turn-on quantum dot probes.

Dimensionality Effects on Fluorescence Resonance Energy Transfer between Single Semiconductor Nanocrystals and Multiple Dye Acceptors

Colloidal semiconductor nanocrystals are outstanding donors in energy transfer processes due to their unique size and shape dependent optical properties, their exceptional photostability, and chemical processability. We examine the dimensionality effect in energy transfer between single heterostructure nanocrystals of spherical and rod shape, serving as donors, and multiple dye molecules attached to their surface acting as acceptors. Förster resonant energy transfer (FRET) to individual dyes attached to the surface of a single nanocrystal is identified via step-like changes in both acceptor and donor emission, enabling to calculate the efficiency of energy transfer and distance of each acceptor individually. This offers a unique tool to study the surface chemistry of various nanocrystals. The dimensionality of the nanocrystals is reflected by the acceptors distribution, which enables to study the inner geometry of these heterostructures, such as the location of the seed and shell thickness. Additionally, the nanocrystals serve as an optical antenna that enhances the excitation and emission of the dye molecules through the FRET interaction. These measurements enable to gain deeper understanding of the energy transfer process between semiconductor nanocrystals of various geometries and dye molecules and promote its utilization for extremely sensitive sensing applications at the single molecule level.

Effects of Post-Synthesis Processing on CdSe Nanocrystals and Their Solids: Correlation between Surface Chemistry and Optoelectronic Properties

In this work, we report the effects on CdSe nanocrystal (NC) surface chemistry of acetone and methanol when used as the antisolvents for NC washing and as the solvents for ligand exchange of NC solids with ammonium thiocyanate (NH4SCN). We find that NCs washed with methanol have significantly fewer remaining organic ligands and lower photoluminescence quantum yield than those washed with acetone. When used as the ligand exchange solvent, methanol leaves more organic ligands and introduces fewer bound thiocyanates on the NC surface than when acetone is used. We demonstrate the effect of these different surface chemistries on NC solid optoelectronic properties through photoconductivity measurements, showing a greater photocurrent in NC solids with greater organic ligand coverage. We also show that NC washing with methanol or ligand exchange with NH4SCN in methanol removes a significant number of surface Cd atoms from the NCs, creating Cd vacancies that act as traps for recombination. Independent of the wash and exchange process, the NC surface may be repaired by introducing CdCl2 to the NC surface, enhancing the measured photocurrent.

Molecular‐Level Switching of Polymer/Nanocrystal Non‐Covalent Interactions and Application in Hybrid Solar Cells

Hybrid composites obtained upon blending conjugated polymers and colloidal semiconductor nanocrystals are regarded as attractive photo­active materials for optoelectronic applications. Here it is demonstrated that tailoring nanocrystal surface chemistry permits to control non‐covalent and electronic interactions between organic and inorganic components. The pending moieties of organic ligands at the nanocrystal surface are shown to not merely confer colloidal stability while hindering charge separation and transport, but drastically impact morphology of hybrid composites during formation from blend solutions. The relevance of this approach to photovoltaic applications is demonstrated for composites based on poly(3‐hexylthiophene) and lead sulfide nanocrystals, considered as inadequate until this report, which enable the fabrication of hybrid solar cells displaying a power conversion efficiency that reaches 3%. By investigating (quasi)steady‐state and time‐resolved photo‐induced processes in the nanocomposites and their constituents, it is ascertained that electron transfer occurs at the hybrid interface yielding long‐lived separated charge carriers, whereas interfacial hole transfer appears hindered. Here a reliable alternative aiming to gain control over macroscopic optoelectronic properties of polymer/nanocrystal composites by mediating their non‐covalent interactions via ligands' pending moieties is provided, thus opening new possibilities towards efficient solution‐processed hybrid solar cells.

Surface Chemistry of CuInS2 Colloidal Nanocrystals, Tight Binding of L-Type Ligands

We analyze the surface chemistry of CuInS2 nanocrystals synthesized in the presence of amines. Using solution NMR spectroscopy and elemental analysis, we come to the conclusion that as-synthesized CuInS2 nanocrystals have charge neutral inorganic cores and are stabilized by a layer of tightly bound L-type amines. In situ NMR heating-up experiments show that desorption of amines can be induced by increasing the temperature, which makes the partial exchange of amines for thiols possible. On the other hand, we find that carboxylic acids are unable to bind as L-type ligands to the CuInS2 surface. In addition, we demonstrate that the use of technical oleylamine in the synthesis of CuInS2 nanocrystals leads to nonstoichiometric nanocrystals which have, next to oleylamine ligands, also X-type impurities on the surface that can be exchanged for carboxylic acids.

Segmented poly(styrene-co-vinylpyridine) as multivalent host for CdSe nanocrystal based nanocomposites

Nanocomposites based on colloidal CdSe nanocrystals (NCs) and a poly(styrene-co-4-vinylpyridine), able to specifically coordinate the NC surface, have been designed and prepared. For first time, the polymer synthesis has been performed by using 2,2,5-tri-methyl-4-phenyl-3-azahexane-3-nitroxide as a mediator, increasing the percentage of 4-vinylpyridine monomeric unit, thus obtaining a random copolymer. The nanocomposite properties have been investigated as a function of NC surface chemistry and copolymer composition, by means of spectroscopic, morphological and structural characterization techniques. An improved uniformity of NC dispersion in the nanocomposite has been found at increased percentage of 4-vinylpyridine in the copolymer. The improved NC dispersion in the nanocomposite films has been discussed in terms of the ability of the copolymer to act as a multivalent ligand. The reported results offer a valuable contribution toward the design and the fabrication of innovative nanocomposite material, formed of copolymers and colloidal NCs, specifically suited for energy conversion applications.

Unravelling the surface chemistry of metal oxide nanocrystals, the role of acids and bases.

We synthesized HfO2 nanocrystals from HfCl4 using a surfactant-free solvothermal process in benzyl alcohol and found that the resulting nanocrystals could be transferred to nonpolar media using a mixture of carboxylic acids and amines. Using solution (1)H NMR, FTIR, and elemental analysis, we studied the details of the transfer reaction and the surface chemistry of the resulting sterically stabilized nanocrystals. As-synthesized nanocrystals are charge-stabilized by protons, with chloride acting as the counterion. Treatment with only carboxylic acids does not lead to any binding of ligands to the HfO2 surface. On the other hand, we find that the addition of amines provides the basic environment in which carboxylic acids can dissociate and replace chloride. This results in stable, aggregate-free dispersions of HfO2 nanocrystals, sterically stabilized by carboxylate ligands. Moreover, titrations with deuterated carboxylic acid show that the charge on the carboxylate ligands is balanced by coadsorbed protons. Hence, opposite from the X-type/nonstoichiometric nanocrystals picture prevailing in literature, one should look at HfO2/carboxylate nanocrystals as systems where carboxylic acids are dissociatively adsorbed to bind to the nanocrystals. Similar results were obtained with ZrO2 NCs. Since proton accommodation on the surface is most likely due to the high Brønsted basicity of oxygen, our model could be a more general picture for the surface chemistry of metal oxide nanocrystals with important consequences on the chemistry of ligand exchange reactions.

Solution-processing of ultra-thin CdTe/ZnO nanocrystal solar cells

We have carried out a detailed study into how modifications of the physical, chemical and optical properties of solution-processed, nanocrystalline CdTe layers influence the photovoltaic performance of sintered CdTe/ZnO nanocrystal solar cells. Such solar cells are fabricated through layer-by-layer assembly, which is enabled through an inter layer chemical and thermal treatment cycle. In this manner we are able to fabricate working solar cells with sintered CdTe layers as low as 90nm, provided that grain size is precisely controlled. We show that the extent of grain growth achieved during the CdTe sintering process is strongly dependent on nanocrystal surface chemistry and chemical environment, with the removal of the organic capping ligands and the introduction of CdCl2 prior to annealing leading to greatly enhanced growth. Due to the air processing involved and the nanocrystalline nature of the CdTe, the overall performance of these solar cells is shown to be strongly dependent on both annealing temperature and time, with optimal results requiring a balance between crystal growth and degradation due to oxidation. Using this simple bi-layer device structure, optimized treatment conditions result in power conversion efficiencies of up to 7.7% and peak internal quantum efficiencies in excess of 95%.

Fluorescence enhancement of CdTe MPA-capped quantum dots by glutathione for hydrogen peroxide determination

The manipulation of the surface chemistry of semiconductor nanocrystals has been exploited to implement distinct sensing strategies in many analytical applications. In this work, reduced glutathione (GSH) was added at reaction time, as an electron-donor ligand, to markedly increase the quantum yield and the emission efficiency of MPA-capped CdTe quantum dots. The developed approach was employed in the implementation of an automated flow methodology for hydrogen peroxide determination, as this can oxidize GSH preventing its surface passivating effect and producing a manifest fluorescence quenching. After optimization, linear working calibration curve for hydrogen peroxide concentrations between 0.0025% and 0.040% were obtained (n=6), with a correlation coefficient of 0.9975. The detection limit was approximately 0.0012%. The developed approach was employed in the determination of H₂O₂ in contact lens preservation solutions and the obtained results complied with those furnished by the reference method, with relative deviations comprised between -1.18 and 4.81%.

Surface chemistry, morphological analysis and properties of cellulose nanocrystals with gradiented sulfation degrees

The process of sulfuric acid-hydrolysis of cellulose fibers for the preparation of cellulose nanocrystals (CNs) includes an esterification reaction between acid and cellulose molecules, which induces the covalent coupling of sulfate groups on the surface of prepared CNs. Negatively charged sulfate groups play an important role in both surface chemistry and physical properties of CNs. This study explored the strategy of introducing a gradient of sulfate groups on the surface of CNs, and further investigated the effect of the sulfation degree on surface chemistry, morphology, dimensions, and physical properties of different CN samples. Based on the discussion of their surface chemistry, the selection of different cross-section models was reported to significantly affect the calculation of the degree of substitution of sulfate groups on CNs. A new ellipsoid cross-section model was proposed on the basis of AFM observations. The effect of sulfate groups on crystal properties and thermal stability was discussed and validated, and the birefringence behavior of nanocrystal suspensions was observed.

Constructing Functional Mesostructured Materials from Colloidal Nanocrystal Building Blocks

Through synthesizing colloidal nanocrystals (NCs) in the organic phase, chemists gain fine control over their composition, size, and shape. Strategies for arranging them into ordered superlattices have followed closely behind synthetic advances. Nonetheless, the same hydrophobic ligands that help their assembly also severely limit interactions between adjacent nanocrystals. As a result, examples of nanocrystal-based materials whose functionality derives from their mesoscale structure have lagged well behind advances in synthesis and assembly. In this Account, we describe how recent insights into NC surface chemistry have fueled dramatic progress in functional mesostructures. In these constructs, intimate contact between NCs as well as with heterogeneous components is key in determining macroscopic behavior. The simplest mesoscale assemblies we consider are networks of NCs constructed by in situ replacement of their bulky, insulating surface ligands with small molecules. Transistors are a test bed for understanding conductivity, setting the stage for new functionality. For instance, we demonstrated that by electrochemically charging and discharging networks of plasmonic metal oxide NCs, the transmittance of near infrared light can be strongly and reversibly modulated. When we assemble NCs with heterogeneous components, there is an even greater potential for generating complex functionality. Nanocomposites can exhibit favorable characteristics of their component materials, yet the interaction between components can also have a strong influence. Realizing such opportunities requires an intimate linking of embedded NCs to the surrounding matrix phase. We accomplish this link by coordinating inorganic anionic clusters directly to NC surfaces. By exploiting this connection, we found enhanced ionic conductivity in Ag2S-in-GeS2 nanocrystal-in-glass electrodes. In another example, we also found enhanced optical contrast when linking electrochromic niobium oxide to embedded tin-doped indium oxide (ITO) NCs. These dramatic effects emerge from reconstruction of the inorganic glass immediately adjacent to the NC interface. When co-assembling NCs with block copolymers, direct coordination of the polymer to NC surfaces again opens new opportunities for functional mesoscale constructs. We strip NCs of their native ligands and design block copolymers containing a NC tethering domain that bonds strongly, yet dynamically, to the resulting open coordination sites. This strategy enables their co-assembly at high volume fractions of NCs and leads to well-ordered mesoporous NC networks. We find these architectures to be exceptionally stable under chemical transformations driven by cation insertion, removal, and exchange. These developments offer a modular toolbox for arranging NCs deliberately with respect to heterogeneous elements and open space. We have control over metrics that define such architectures from the atomic scale (bonding and crystal structure) through the mesoscale (crystallite shapes and sizes and pore dimensions). By tuning these parameters and better understanding the interactions between components, we look forward to boundless opportunities to employ mesoscale structure, in tandem with composition, to develop functional materials.

Surfaces of colloidal PbSe nanocrystals probed by thin-film positron annihilation spectroscopy

Positron annihilation lifetime spectroscopy and positron-electron momentum density (PEMD) studies on multilayers of PbSenanocrystals (NCs),supported by transmission electron microscopy, show that positrons are strongly trapped at NC surfaces, where they provide insight into the surface composition and electronic structure of PbSeNCs. Our analysis indicates abundant annihilation of positrons with Se electrons at the NC surfaces and with O electrons of the oleic ligands bound to Pb ad-atoms at the NC surfaces, which demonstrates that positrons can be used as a sensitive probe to investigate the surface physics and chemistry of nanocrystals inside multilayers. Ab initio electronic structure calculations provide detailed insight in the valence and semi-core electron contributions to the positron-electron momentum density of PbSe. Both lifetime and PEMD are found to correlate with changes in the particle morphology characteristic of partial ligand removal.