Highly Oriented Pyrolytic Graphite Substrate Research Articles

Guest-induced structural phase transformations of the multicomponent network at liquid/solid interfaces

The fabrication of bi-component and three-component supramolecular networks based on hydrogen bonds was investigated by scanning tunnelling microscope (STM). Induced by successively introduced coronene (COR) and 1,3,5-benzenetricarboxylic acid (TMA), the intermolecular interactions between [1,1′-biphenyl]-3,4′,5-tricarboxylicacid (BPTC) were changed, leading to the structural transformation of the lamellar self-assembled nanostructure of BPTC on highly oriented pyrolytic graphite (HOPG). Eventually, the flower-like nanostructure of BPTC/COR and kagomé nanostructure of BPTC/TMA/COR were obtained on HOPG substrates respectively. This investigation of the multicomponent host–guest network could be useful for the construction of novel heterogenous molecular structures.

Preparation of Electrochemical Sensor for Paracetamol Detection Based on a New Piperazine Substituted Triazole-Coumarin Hybrid Doped PEDOT Modified Flexible HOPG Electrode

This study includes the electrochemical determination of paracetamol (PC), using novel piperazine substituted triazole-coumarin hybrids doped PEDOT coated highly oriented pyrolytic graphite (HOPG) electrode. For the electrochemical determination of paracetamol, using an electrode prepared by a one step electropolymerization of the new triazole-tethered piperazine coumarin hybrid doped PEDOT conductive polymer on the HOPG substrate was mentioned. The electrodes were characterized by FT-IR, SEM and XRD analysis. The possible interference effects of Ascorbic acid, Uric acid, Dopamine, Glucose and Caffeine were investigated by chronoamperometry in the presence of paracetamol. The sensor (PEDOT-MC1/HOPG-10 Scan) has a high sensitivity to paracetamol, with a limit of detection (LOD) of 0.312 μm and a RSD % of 1.22 (S/N = 3). In addition, real sample analyzes were performed on pharmaceutical samples for PC determination using the prepared sensor (PEDOT-MC1/HOPG-10 Scan). The novelty of this paper is the use of new piperazine substituted triazole-coumarin hybrid doped PEDOT material for the first time in the literature, as well as a new electrode substrate material for the determination of paracetamol and the use of HOPG (Highly oriented pyrolytic graphite), which exhibits higher electroanalytical performance than other carbon-based materials.

Comprehensive characterization of different metallic thin films on highly oriented pyrolytic graphite substrate

Thin metallic films supported on highly oriented pyrolytic graphite (HOPG) with low surface roughness are needed to improve the performance of different industrial applications and devices. As a result, an experimental investigation of electron beam physical vapour deposition of M (= Ti, Au, and Ag) metals on HOPG substrate to form HOPG-M thin films with a nominal thickness of 10 nm is reported. The comprehensive characterizations of the films were performed using different characterization techniques. The findings show that the films formed had different morphologies and topographies where Ti film was continuous while Au and Ag films were made up of islands with some discontinuities. The surface chemistry shows that the Ti film existed in both metallic and oxidized states while the Au and Ag films existed in metallic states. Raman analysis shows that there were some variations in the molecular structures after deposition while FTIR analysis shows that different functional groups existed on the thin films. The surface wettability behaviour of the Ag film based on water contact angles was hydrophobic while the other samples were hydrophilic. This work provides an opportunity for further examination of the HOPG as a substrate for the preparation of different M thin films.

Substrate Effect on Band Bending of MoSe2 Monolayer Near Mirror‐Twin Domain Boundaries

AbstractBand bending near mirror twin domain boundaries (MTBs) in a MoSe2 monolayer grown on different substrates, i.e., highly oriented pyrolytic graphite (HOPG), graphene‐on‐SiC, and crystalline Au(110), is investigated by low temperature scanning tunneling microscopy/spectroscopy. Upshift bending of the valence band edge near MTB is observed on both graphene and HOPG substrates, whereas a downshift bending is found on Au(110). For the former, the magnitudes of bending are different. This is explained based on the static charge model, where an accumulative charge exists at MTBs due to both the electrical dipole discontinuity across the MTB as well as charge transfer between the substrate and MoSe2 epilayer. The relevance of the static electric model is further affirmed by noting a geometric effect on the band bending, where it is asymmetric across the vertex of an MTB loop.

Comparative study of electrodeposited copper nanoparticles on different substrates for their use in the reduction of nitrate ions

A comparative analysis of the electrodeposition of copper nanoparticles (CuNPs) was studied on highly oriented pyrolytic graphite (HOPG) and vitreous carbon (VC) substrates from solutions containing different supporting electrolytes (Na2SO4 and H2SO4).Voltammetric results indicated the presence of a single cathodic process, corresponding to the reduction reaction of Cu2+ ions to Cuo, for the solutions studied, using the HOPG substrate. The solution containing Na2SO4 as supporting electrolyte was found to be more effective because the copper reduction process occurs at more positive potential values, reaching the highest current density. For the VC substrate, two cathodic processes were evidenced, with the formation of Cu+ ions as intermediate species. As in the HOPG substrate, the use of the solution containing Na2SO4 showed the better behavior for copper electrodeposition, being the most favored process on VC. This behavior could be explained by considering the existence of structural defects in the VC, which would facilitate Cu nucleation.The nucleation and growth kinetics of CuNPs on HOPG electrodes corresponded predominantly to a model including progressive nucleation on active sites, and diffusion controlled growth, presenting a good correlation with the scanning electron microscopy images. For VC substrates, the kinetics analysis did not yield conclusive results. Different morphological features of Cu deposits were observed on HOPG and VC, presenting the former a larger covered area.The HOPG/CuNPs modified electrodes evidenced an enhancement in the catalytic activity towards the nitrate reduction reaction.

Frictional shear stress of ZnO nanowires on natural and pyrolytic graphite substrates

The friction behaviour of ZnO nanowires on natural graphite (NG) and highly oriented pyrolytic graphite (HOPG) substrates was tested in ambient conditions by use of optical microscopy based nanomanipulation. Nanowires on the step-free and waviness-free NG substrate exhibit a diameter-independent nominal frictional shear stress of 0.48 MPa, and this provides a benchmark for studying how the surface topography of graphite influences nanowire friction. Nanowires on the HOPG substrate present a significant diameter-dependent frictional shear stress, increasing from 0.25 to 2.78 MPa with the decrease of nanowire diameter from 485 to 142 nm. The waviness of HOPG has a limited effect on the nanowire friction, as a nanowire can fully conform to the substrate. The surface steps on the HOPG can significantly enhance the nanowire friction and lead to a much higher frictional shear stress than that on NG due to mechanical blocking and the presence of a Schwoebel barrier at step edges. The surface steps, however, can also generate small wedge-shaped gaps between a nanowire and substrate, and thus reduce the nanowire friction. With the decrease in nanowire diameter, the capacity for the nanowire to better conform to the substrate reduces the length of the wedge-shaped gaps, leading to the observed increase in nanowire friction. The results have improved our understanding of the unique friction behaviour of nanowires. Such an improved understanding is expected to benefit the design and operation of nanowire-friction-based devices, including bio-inspired fibrillar adhesives, soft grippers, rotary nanomotors, and triboelectric nanogenerators.

Model Catalysis with HOPG-Supported Pd Nanoparticles and Pd Foil: XPS, STM and C2H4 Hydrogenation

A surface science based approach was applied to model carbon supported Pd nanoparticle catalysts. Employing physical vapour deposition of Pd on sputtered surfaces of highly oriented pyrolytic graphite (HOPG), model catalysts were prepared that are well-suited for characterization by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Analysis of the HOPG substrate before and after ion-bombardment, and of Pd/HOPG before and after annealing, revealed the number of “nominal” HOPG defects (~ 1014 cm−2) as well as the nucleation density (~ 1012 cm−2) and structural characteristics of the Pd nanoparticles (mean size/height/distribution). Two model systems were stabilized by UHV annealing to 300 °C, with mean Pd particles sizes of 4.3 and 6.8 nm and size/height aspect ratio up to ~ 10. A UHV-compatible flow microreactor and gas chromatography were used to determine the catalytic performance of Pd/HOPG in ethylene (C2H4) hydrogenation up to 150 °C under atmospheric pressure, yielding temperature-dependent conversion values, turnover frequencies (TOFs) and activation energies. The performance of Pd nanocatalysts is compared to that of polycrystalline Pd foil and contrasted to Pt/HOPG and Pt foil, pointing to a beneficial effect of the metal/carbon phase boundary, reflected by up to 10 kJ mol−1 lower activation energies for supported nanoparticles.Graphical

Lateral Distribution of Optical Excitation at Boundaries around Rubrene Islands Visualized by Microspot Two-Photon Photoemission Spectroscopy

The lateral distribution of optical excitation occurring at the edge part of rubrene (5,6,11,12-tetraphenyltetracene) molecular islands formed on highly oriented pyrolytic graphite (HOPG) substrate has been visualized with microspot two-photon photoemission (2PPE) spectroscopy of 0.4 µm resolution. As we reported previously [T. Ueba, et al., J. Phys. Chem. C 117 (2013) 20098–20103], an unoccupied peak labeled Ln is strongly enhanced in 2PPE spectroscopy by a resonant optical excitation between a diffuse unoccupied Ln molecular orbital and the HOMO. The strong enhancement is due to the hybridization of Ln with the image potential state (IPS) on the HOPG substrate. In this study, using the spatially-resolved microspot 2PPE method, we obtained the real space image of the area where the optical excitation occurs. One-photon photoelectron emission microscope (1PPE-PEEM) with 50 nm resolution is used to compare with the microspot 2PPE. The 1PPE-PEEM images are homogeneous for films below and above 1 monolayer (ML) coverage, indicating that the molecular density of rubrene is homogeneous within the lateral resolution. On the other hand, microspot 2PPE images for sub-ML films show inhomogeneous areas of several µm sizes. The images depend on the detected unoccupied levels of interest. The inhomogeneity is very significant when the image is recorded at the energy of Ln peak. The different appearance between 1PPE-PEEM and microspot 2PPE images is explained on the basis that the Ln excitation occurs at the edges of islands smaller than the resolution of the PEEM. Though the microspot 2PPE achieves only moderate lateral resolution, it is sensitive to the resonant excitation occurring at nm-scale boundaries of the molecular islands. The distribution of the nm-scale islands is the origin of the µm-scale lateral inhomogeneity. The microspot 2PPE images become homogeneous for films thicker than 1 ML, reflecting the quenching of the resonant excitation. These results demonstrate a possibility that spatially- and energy-resolved microspot 2PPE can be a powerful tool to clarify nanoscale geometric and electronic structures at the molecule/substrate interfaces.

Незаполненные электронные состояния и потенциальный барьер в пленках замещенных дифенилфталидов на поверхности высокоупорядоченного пиролитического графита

The results of a study of the unoccupied electronic states of ultrathin films of bis-carboxyphenyl-phthalide (DCA-DPP) and bis-methylphenyl-phthalide (DM-DPP) up to 8 nm thick are presented. The studies were carried out by total current spectroscopy (TCS) technique in the energy range from 5 eV to 20 eV above EF during thermal vacuum deposition of these organic films on the surface of highly oriented pyrolytic graphite (HOPG). The energy Evac relative to EF, that is, the electronic work function of the DM-DPP films, at a film thickness of 5–8 nm was 4.3 ± 0.1 eV. The electronic work function of the DCA-DPP films was 3.7 ± 0.1 eV. The structure of the maxima of the unoccupied electronic states of DCA-DPP films and DM-DPP films in the studied energy range is determined. The properties determined of DCA-DPP and DM-DPP films are compared with the properties of films of unsubstituted diphenylphthalide (DPP). According to our analysis, –CH3 substitution of the DPP molecule practically did not affect the height of the potential barrier between the film and the HOPG surface, and –COOH substitution of the DPP molecule led to an increase in the height of the potential barrier between the film and the HOPG substrate surface by 0.5–0.6 eV. Substitution of DPP molecules with –COOH functional groups which represents formation of DCA-DPP molecules led to a shift of two peaks of the experimental total current spectra located at energies in the range from 5 eV to 8 eV above EF, by about 1 eV towards lower electron energies.

Evolution of Electronic Structures of Polar Phthalocyanine-Substrate Interfaces.

The electronic structures and core-level spectra of chlorogallium phthalocyanine (ClGaPc) molecules of different thicknesses (submonolayer to multilayer) adsorbed on a polycrystalline Au substrate and a highly oriented pyrolytic graphite (HOPG) substrate, before and after thermal annealing, were investigated using photoelectron spectroscopic techniques for better understanding the charge-transfer properties. The energy-level diagrams (ELDs) of the ClGaPc thin films are found to evolve with film thickness, substrate nature, and thermal annealing. The interfacial dipole moment in the active Au substrate and the molecular dipole moment in the inactive HOPG substrate mainly dictate the ELD. Annealed monolayer films on both the substrates seem to adopt a similar well-ordered Cl-up orientated molecular organization, which is quite interesting, as it certainly indicates a substrate-nature-independent energy minimum configuration. The strong interaction of the active Au substrate gives rise to additional charge transfer and state transfer (of Ga) as evident from the formation of a former lowest unoccupied molecular orbital (F-LUMO) level in the highest occupied molecular orbital (HOMO) region and a low binding energy peak in the Ga 2p3/2 core level. The presence of strong F-LUMO and molecular-dipole-related HOMOd levels in the predicted monolayer of well-ordered Cl-up oriented molecules on the Au and HOPG substrates, respectively, creates the optimum energy-level alignment (ELA) for both the systems, while the opposite shift of the vacuum levels in two different substrates makes the ionization potential (IP) for such a monolayer either minimum (on the Au substrate) or maximum (on the HOPG substrate), which is useful information for tuning the charge injection across the interface in organic semiconductor-based devices.

Cross‐sectional structural characterization of the surface of exfoliated HOPG using HRTEM‐EELS

We analyzed the surface atomic structure of highly oriented pyrolytic graphite (HOPG) substrate exfoliated with adhesive tape, using high‐resolution transmission electron microscopy and scanning transmission electron microscopy‐electron energy‐loss spectroscopy (STEM‐EELS). The surface step height of the exfoliated HOPG substrate was determined using high‐angle annular dark‐field‐scanning transmission electron microscopy (HAADF‐STEM) images and the depth profiles of the EELS spectra of a cross‐sectioned thin foil specimen prepared via focused ion beam milling. The exfoliated surface of the HOPG substrate presented disordered and curved graphene layers. The STEM‐EELS measurements indicated that upon exfoliation, the surface of the HOPG substrate reacted with atmospheric water and oxygen molecules.

Complementary SEM-AFM of Swelling Bi-Fe-O Film on HOPG Substrate

The objective of this work is to study the delamination of bismuth ferrite prepared by atomic layer deposition on highly oriented pyrolytic graphite (HOPG) substrate. The samples’ structures and compositions are provided by XPS, secondary ion mass spectrometry (SIMS) and Raman spectroscopy. The resulting films demonstrate buckling and delamination from the substrates. The composition inside the resulting bubbles is in a gaseous state. It contains the reaction products captured on the surface during the deposition of the film. The topography of Bi-Fe-O thin films was studied in vacuum and under atmospheric conditions using simultaneous SEM and atomic force microscopy (AFM). Besides complementary advanced imaging, a correlative SEM-AFM analysis provides the possibility of testing the mechanical properties by using a variation of pressure. In this work, the possibility of studying the surface tension of the thin films using a joint SEM-AFM analysis is shown.

Transferable High-Quality Inorganic Perovskites for Optoelectronic Devices by Weak Interaction Heteroepitaxy.

Transferable semiconductors with superior light-emitting properties are important for developing flexible and integrated optoelectronics. However, finding such a qualified candidate remains challenging. Here, we report the fabrication of transferable high-quality CsPbBr3 single crystals on a highly oriented pyrolytic graphite (HOPG) substrate via weak interaction heteroepitaxy for the first time. Semi-quantitative kinetic analysis based on the classical nucleation theory well accounts for the van der Waals (vdW) epitaxial growth process of perovskite on the HOPG substrate. The density functional theory calculations illustrate the bonding nature of the interface and predict the Volmer-Weber growth mode in vdW epitaxy, which is consistent with our experimental observations. Importantly, the extremely weak vdW interaction between the perovskite and HOPG not only enables the high quality of the crystals but also endows them with the facile transferability to any foreign substrate by the mechanical exfoliation technique. Leveraging on the transferred CsPbBr3 single crystals, the low-threshold microlasers and monolithic perovskite light-emitting diode devices are demonstrated. Our results represent a significant step toward advanced optoelectronic devices relying on the emerging perovskite semiconductors.

Effects of Mn doping on BaTiO3 thin films grown on highly oriented pyrolytic graphite substrates

We report multiferroelectric properties of Mn-doped BaTiO3 (MBTO) thin films on highly oriented pyrolytic graphite (HOPG) substrates. The MBTO thin films were grown on the HOPG substrate by pulse laser deposition. For comparison purpose, undoped BaTiO3 (BTO) thin films were also prepared under same experimental conditions. The BTO and MBTO thin films were polycrystalline, indicating that the MBTO thin film has better crystallinity than the BTO thin film. The leakage current of the MBTO thin film was reduced due to the Mn doping substitution. In addition, the MBTO thin film exhibited better than the BTO thin film in ferroelectric and magnetic behaviors. We suggest that the Mn doping bring about the improvements of ferroelectric and ferromagnetic properties of the BTO thin films. Based on atomic force microscopy (AFM) and conducting AFM (CAFM) studies, the grain size of MBTO thin film was much larger than that of BTO thin film.

Controlled ultra-thin oxidation of graphite promoted by cobalt oxides: Influence of the initial 2D CoO wetting layer

The interaction of CoO with highly oriented pyrolytic graphite (HOPG) was studied using a set of complementary techniques. The morphology of the CoO thin film was determined using atomic force microscopy (AFM), whereas the electronic structure was investigated using X-ray absorption (XAS) and photoemission (PES) spectroscopies. The experimental spectra were analyzed using a configuration interaction CoO6 cluster model calculation. The early stages of growth are characterized by the formation of a CoO wetting layer at the CoO/HOPG interface. The electronic structure of the CoO wetting layer presents a clear 2D character, which is closer to the 2D HOPG substrate than to the 3D CoO bulk. This character of the wetting layer explains the posterior formation of CoO islands and excludes the alternative layer by layer growth mode. Further, the interaction between the CoO wetting layer and the outermost graphite layer favors the oxidation of the HOPG substrate which can be controlled by the thickness of the deposited CoO overlayer.

HRTEM-EELS cross-sectional characterization of HOPG substrate with platinum nanoparticles deposited using a coaxial arc plasma gun

Platinum nanoparticles were deposited on a highly oriented pyrolytic graphite (HOPG) substrate using a coaxial arc plasma gun. The Pt-deposited HOPG substrate was cross-sectioned to prepare a thin foil specimen using focused ion beam milling, and was analyzed by using high resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy. The surface of the HOPG substrate was observed to be disordered by the Pt ion irradiation. The surface was located using the depth profiles of π + σ plasmon peaks and the full width at half maximum of π⁎ peaks. The denaturation of the HOPG substrate by the Pt ion irradiation was observed to a depth of approximately 12 nm.

One-Step Covalent Immobilization of β-Cyclodextrin on sp2 Carbon Surfaces for Ultrasensitive and Selective Guest Detection

The modification of solid surfaces with supramolecular hosts is a powerful method to tailor interfacial properties and confer chemical selectivity, but often involves multistep protocols that hinder facile upscaling. Here, we demonstrate the one-step covalent modification of highly oriented pyrolytic graphite (HOPG) with a β-cyclodextrin (β-CD) derivative, which efficiently forms inclusion complexes with hydrophobic guests of suitable size (e.g. ferrocene, dopamine). The modified β-CD-HOPG electrode discriminates analytes that form host-guest complexes with β-CD against a 100-fold higher background of electroactive substances that do not, even when these possess similar redox potentials. In addition, the enrichment of the analytes at the electrode surface, through complex formation with the β-CD units, leads to an enhanced electrochemical response and an improved detection limit (one order of magnitude for dopamine, compared to current cyclodextrin-based sensors). The two unique features we demonstrate for the covalently grafted β-CD-HOPG electrode (selective sensing of hydrophobic analytes and ultrasensitive dopamine detection) do not come at the expense of ease of preparation: the one-step covalent modification can be completed in minutes, using standard electrochemical cells and equipment. Also, the procedure uses robust, commercially available HOPG substrates that can be reused by mechanical exfoliation many times, lowering the operational cost and further increasing wide accessibility and attractiveness of our approach. The fundamental reasons for the high performance of the sensor may be related to its simple and well-defined structure, in which all the sensing units (covalently attached β-CD-moieties) are positioned uniformly and close to the collector surface (HOPG). This is achieved by the chemical characteristics of both reagent (a highly reactive, but sterically hindered diazonium compound, yielding monolayer modification exclusively) and substrate (low-reactive sp2 carbon surface, requiring highly reactive radical reagents for its modification), resulting in negligible side reactions despite the simplicity of the approach. These principles, and the resulting performance, can be taken as a proof of concept for entirely different sensors, e.g. using other van der Waals solids as substrates, but based on analogous chemistries.

1-nm-Wide Hydrated Dipole Arrays Regulate AuNW Assembly on Striped Monolayers in Nonpolar Solvent

Summary We show that striped phases of horizontally oriented phospholipids presenting 1-nm-wide orientable dipole arrays can order and straighten flexible 2-nm-diameter gold nanowires (AuNWs) with lengths (up to 1 μm) that greatly exceed the template pitch (∼7.5 nm). AuNW ordering can extend over areas > 100 μm2. Whereas wires bundle in solution, with contact between ligand shells, wires adsorbing to the template separate to much larger center-to-center distances (e.g., 12–30 nm) near multiples of the template pitch, consistent with repulsive interactions between wires emerging during interactions with dipole arrays in the template. AuNW assembly was carried out in cyclohexane, a nonpolar solvent. Interestingly, we found that wire ordering was contingent on the extent of phospholipid headgroup hydration and the presence of excess oleylamine, which assembled into hemicylindrical micelles around the hydrated headgroups. Together, these components generated a protected polar environment that enabled the phospholipid headgroups to function collectively to regulate AuNW assembly.

Effects of gas cluster ion beam sputtering on the molecular orientation of organic semiconductor films: Ultraviolet photoelectron spectroscopy study of [6]phenacene

The alignment of organic layer and metal layer energy levels within organic semiconductor devices is critical for efficient charge injection/collection at the electrodes. The electronic structure at the interface is usually examined using ultraviolet photoelectron spectroscopy (UPS) as the film thickness of the organic layer is built up incrementally (bottom-up method). As such, the morphology and crystallinity of the films that are examined using this technique may be different from the films used in actual devices because these properties are sensitive to the way in which the films are prepared. We have prepared [6]phenacene thin films with standing and lying molecular orientations on naturally oxidized silicon (SiO2) and highly oriented pyrolytic graphite (HOPG) surfaces, respectively. UPS measurements were performed on films that were incrementally decreased in thickness using gas cluster ion beam (GCIB) sputtering (top-down method). The spectral shapes and ionization energies of the films that were formed using the bottom-up and top-down methods were similar. Importantly, the characteristic features of a monolayer on the HOPG substrate were also observed when a thick film (i.e., not built up in layers using the bottom-up method) was thinned down by GCIB sputtering. We have shown that UPS combined with GCIB sputtering is a useful technique for examining the energy level alignment of interfaces within films that are fabricated using conditions similar to those used for real devices.

Doped Colloidal InAs Nanocrystals in the Single Ionized Dopant Limit

We investigate the electronic properties of individual n-type (Cu) doped and p-type (Ag) doped InAs colloidal nanocrystals (NCs) in the 2–8 nm size range from their charge transfers toward a highly oriented pyrolytic graphite (HOPG) substrate, using ultrahigh vacuum Kelvin probe force microscopy (KPFM) with elementary charge sensitivity at 300 K. The NC active dopant concentration is measured as ND = 8 × 1020 cm–3 and NA > 5 × 1020 cm–3 for n- and p-type doping, respectively. The electrostatic equilibrium between the NC and the HOPG reference substrate is investigated and reveals an enhancement of the Fermi-level mismatch between the NCs and the HOPG substrate at reduced NC sizes, both for n- and p-type doping. It also shows, for n-type doped NCs with smallest sizes (∼2 nm), the existence of a full depletion regime, in which smallest NCs contain single ionized dopants. Results are compared with self-consistent tight-binding calculations of the electronic structure of InAs NCs, including hydrogenoid impurities and the presence of a host substrate, in the case of n-type doped NCs. The observed enhancement of the NC–HOPG Fermi-level mismatch can be understood by considering a size-dependent electrostatic contribution attributed to dipolar effects at the NC–ligand interface. The estimated surface dipole density equals a few Debye/nm2 and is increased at smallest NC sizes, which follows the enhancement of ligand densities at small NC sizes previously reported for metallic or semiconducting NCs. The results put forward the role played by the NC–ligand interface electrostatics for electronic applications.