Photovoltaic and photothermal effects induced by visible laser radiation in atomic force microscopy probes

Excitation of electrons into higher energy states in solid state materials can be induced by absorption of visible light, a physical process generally studied by optical absorption spectroscopy. A promising approach for improving the spatial resolution of optical absorption spectroscopy beyond the diffraction limit is the detection of photoinduced forces by an atomic force microscope operating under wavelength-dependent light irradiation. Here, we report on a combined photovoltaic/photothermal effect induced by the absorption of visible light by the microscope probes. By monitoring the photoinduced modifications of the oscillation of the probes, it is found that the oscillation phase-voltage parabolic signals display specific fingerprints which depend on light intensity and the nature of the materials composing the probes. In particular, a localized surface photovoltage (SPV) is evidenced at the tip apex of uncoated Si probes, while none is observed on Au-coated Si probes. The photothermal effects are distinguished from photovoltaic effects by specific shifts of the phase-voltage parabolas. The findings are relevant for the whole range of atomic force microscopy techniques making use of visible light as an additional means of local optical characterization.

Photonic Excitation of a Micromechanical Cantilever in Electrostatic Fields

We present a specific near-field configuration where an electrostatic force gradient is found to strongly enhance the optomechanical driving of an atomic force microscope cantilever sensor. It is shown that incident photons generate a photothermal effect that couples with electrostatic fields even at tip-surface separations as large as several wavelengths, dominating the cantilever dynamics. The effect is the result of resonant phenomena where the photothermal-induced parametric driving acts conjointly (or against, depending on electric field direction) with a photovoltage generation in the cantilever. The results are achieved experimentally in an atomic force microscope operating in vacuum and explained theoretically through numerical simulations of the equation of motion of the cantilever. Intrinsic electrostatic effects arising from the electronic work-function difference of tip and surface are also highlighted. The findings are readily relevant for other optomicromechanical systems where electrostatic force gradients can be implemented.

Magnetic force fields of isolated small nanoparticle clusters

The usage of magnetic nanoparticles (NPs) in applications necessitates a precise mastering of their properties at the single nanoparticle level. There has been a lot of progress in the understanding of the magnetic properties of NPs, but incomparably less when interparticle interactions govern the overall magnetic response. Here, we present a quantitative investigation of magnetic fields generated by small clusters of NPs assembled on a dielectric non-magnetic surface. Structures ranging from individual NPs to fifth-fold particulate clusters are investigated in their magnetization saturation state by magnetic force microscopy and numerical calculations. It is found that the magnetic stray field does not increase proportionally with the number of NPs in the cluster. Both measured and calculated magnetic force fields underline the great importance of the exact spatial arrangement of NPs, shedding light on the magnetic force field distribution of particulate clusters, which is relevant for the quantitative evaluation of their magnetization and perceptibly for many applications.

Nonlinear Phase Imaging of Gold Nanoparticles Embedded in Organic Thin Films

The phase detection in the dynamic mode of the atomic force microscopes is a known technique for mapping nanoscale surface heterogeneities. We present here an additional functionality of this technique, which allows high-resolution imaging of embedded inorganic nanoparticles with diameter and interparticle distances of a few nanometers. The method is based on a highly nonlinear tip-sample interaction occurring markedly above the nanoparticles, giving thus a high phase contrast between zones with and without nanoparticles. A relationship between the tip-sample interaction strength and the phase signal is established in experiments and from calculations conducted with the model developed by Haviland et al. [ Soft Matter 2016 , 12 , 619 ], which is based on solving a combined equation of motion for both the cantilever and surface while taking into account the time-varying interaction forces. The nonlinear phase behavior at the origin of the subnanometer spatial resolution is found by numerical analyses to be the result of a local mechanical stiffening of the zone containing nanoparticles, which is enhanced by 2 orders of magnitude or more.

Interfacial Behavior of Oligo(Ethylene Glycol) Dendrons Spread Alone and in Combination with a Phospholipid as Langmuir Monolayers at the Air/Water Interface

Dendrons consisting of two phosphonate functions and three oligo(ethylene glycol) (OEG) chains grafted on a central phenoxyethylcarbamoylphenoxy group were synthesized and investigated as Langmuir monolayers at the surface of water. The OEG chain in the para position was grafted with a t-Bu end-group, a hydrocarbon chain, or a partially fluorinated chain. These dendrons are models of structurally related OEG dendrons that were found to significantly improve the stability of aqueous dispersions of iron oxide nanoparticles when grafted on their surface. Compression isotherms showed that all OEG dendrons formed liquid-expanded Langmuir monolayers at large molecular areas. Further compression led to a transition ascribed to the solubilization of the OEG chains in the aqueous phase. Brewster angle microscopy (BAM) provided evidence that the dendrons fitted with hydrocarbon chains formed liquid-expanded monolayers throughout compression, whilst those fitted with fluorinated end-groups formed crystalline-like domains, even at large molecular areas. Dimyristoylphosphatidylcholine and dendron molecules were partially miscible in monolayers. The deviations to ideality were larger for the dendrons fitted with a fluorocarbon end-group chain than for those fitted with a hydrocarbon chain. Brewster angle microscopy and atomic force microscopy supported the view that the dendrons were ejected from the phospholipid monolayer during the OEG conformational transition and formed crystalline domains on the surface of the monolayer.

High-resolution manipulation of gold nanorods with an atomic force microscope

The controlled manipulation and precise positioning of nanoparticles on surfaces is a critical requisite for studying interparticle interactions in various research fields including spintronics, plasmonics, and nanomagnetism. We present here a method where an atomic force microscope operating in vacuum is used to accurately rotate and displace CTAB-coated gold nanorods on silica surfaces. The method relies on operating an AFM in a bimodal way which includes both dynamic and contact modes. Moreover, the phase of the oscillating probe is used to monitor the nanoparticle trajectory, which amplitude variations are employed to evaluate the energy dissipation during manipulation. The nanoscale displacement modes involve nanorod in-plane rotation and sliding, but no rolling events. The transitions between these displacement modes depend on the angle between the scan axis direction and the nanorod long axis. The findings reveal the importance of mean tip-substrate distance and of oscillation amplitude of the tip. The role of substrate surface and of CTAB molecular bi-layer at nanorod surface is also discussed.

Interface bonding in silicon oxide nanocontacts: interaction potentials and force measurements

The interface bonding between two silicon-oxide nanoscale surfaces has been studied as a function of atomic nature and size of contacting asperities. The binding forces obtained using various interaction potentials are compared with experimental force curves measured in vacuum with an atomic force microscope. In the limit of small nanocontacts (typically <10³ nm²) measured with sensitive probes the bonding is found to be influenced by thermal-induced fluctuations. Using interface interactions described by Morse, embedded atom model, or Lennard-Jones potential within reaction rate theory, we investigate three bonding types of covalent and van der Waals nature. The comparison of numerical and experimental results reveals that a Lennard-Jones-like potential originating from van der Waals interactions captures the binding characteristics of dry silicon oxide nanocontacts, and likely of other nanoscale materials adsorbed on silicon oxide surfaces. The analyses reveal the importance of the dispersive surface energy and of the effective contact area which is altered by stretching speeds. The mean unbinding force is found to decrease as the contact spends time in the attractive regime. This contact weakening is featured by a negative aging coefficient which broadens and shifts the thermal-induced force distribution at low stretching speeds.

Nanoscale adhesion and sliding on biased semiconductors

We present experimental and theoretical results on controlling nanoscale sliding friction and adhesion by electric fields on model contacts realized by bringing a conductive atomic force microscope tip into contact with the surface of a silicon-oxide/silicon wafer. We find that applying a bias voltage on silicon (or on the conductive tip) enables a noticeable control of the sliding forces. Two electrostatic interactions are identified as being relevant for the friction variation as a function of applied voltage. The first is a short-range electrostatic interaction between opposite charges localized at oxide-silicon/silicon and tip/silicon-oxide interfaces. This attractive interaction results from the high capacity of the oxide-semiconductor interface to change its charge density in response to a bias voltage. Various regimes of charging resulting from silicon electronic bands' alignment and deformation are evidenced. We mainly focused here on the strong charge accumulation and inversion domains. The second longer-range electrostatic interaction is between the voltage-induced bulk and surface charges of both tip and sample. This interaction decreases very slowly with the distance between tip and silicon surface, i.e. oxide thickness, and can be attractive or repulsive depending on voltage polarity. Our results demonstrate the possibility of controlling nanoscale friction/adhesion in nanoscale contacts involving semiconductors. These results are relevant for the operation of nanoscale devices or for on-surface nanomanipulation of metallic nanoparticles. We model the experimental results by adding an electrostatic energy contribution to the tip-surface binding energy, which translates into an increase or decrease of the normal force and ultimately of the sliding friction.

Stochastic stick-slip nanoscale friction on oxide surfaces

The force needed to move a nanometer-scale contact on various oxide surfaces has been studied using an atomic force microscope and theoretical modeling. Force-distance traces unveil a stick-slip movement with erratic slip events separated by several nanometers. A linear scaling of friction force with normal load along with low pull-off forces reveals dispersive adhesive interactions at the interface. We model our findings by considering a variable Lennard-Jones-like interaction potential, which accounts for slip-induced variation of the effective contact area. The model explains the formation and fluctuation of stick-slip phases and provides guidelines for predicting transitions from stick-slip to continuous sliding on oxide surfaces.

Tailoring Anisotropic Interactions between Soft Nanospheres Using Dense Arrays of Smectic Liquid Crystal Edge Dislocations

We investigated composite films of gold nanoparticles (NPs)/liquid crystal (LC) defects as a model system to understand the key parameters, which allow for an accurate control of NP anisotropic self-assemblies using soft templates. We combined spectrophotometry, Raman spectroscopy, and grazing incidence small-angle X-ray scattering with calculations of dipole coupling models and soft sphere interactions. We demonstrate that dense arrays of elementary edge dislocations can strongly localize small NPs along the defect cores, resulting in formation of parallel chains of NPs. Furthermore, we show that within the dislocation cores the inter-NP distances can be tuned. This phenomenon appears to be driven by the competition between "soft (nano)sphere" attraction and LC-induced repulsion. We evidence two extreme regimes controlled by the solvent evaporation: (i) when the solvent evaporates abruptly, the spacing between neighboring NPs in the chains is dominated by van der Waals interactions between interdigitated capping ligands, leading to chains of close-packed NPs; (ii) when the solvent evaporates slowly, strong interdigitation between the is avoided, leading to a dominating LC-induced repulsion between NPs associated with the replacement of disordered cores by NPs. The templating of NPs by topological defects, beyond the technological inquiries, may enable creation, investigation, and manipulation of unique collective features for a wide range of nanomaterials.

Super-elastic air/water interfacial films self-assembled from soluble surfactants

We show that water-soluble monosodic salts of F-alkyl phosphates C(n)F(2n+1) (CH2)2OP(O)(OH)2, with n=8 and 10 (F8H2Phos and F10H2Phos) form Gibbs films with exceptionally high dilational viscoelastic modules E that reach ~900 mN m(-1) in the condensed phases. These E values are up to one order of magnitude larger than those recorded for phospholipid, protein and polymer films commonly considered as highly viscoelastic. F8H2Phos.1Na undergoes a transition between a liquid-expanded and a liquid-condensed phase. In the case of F10H2Phos.1Na, a transition occurs between a gas phase of surface domains, in which the molecules are densely packed, and a liquid-condensed phase.

Puckering stick-slip friction induced by a sliding nanoscale contact

An atomic force microscope reveals that the sliding of a nanotip on a graphite surface occurs through a nanoscale stick-slip mechanism. The angle between the sliding direction and a stiff crystallographic axis determines the periodicity of the slip events defining domains of various friction properties. The experimental data are interpreted using the reaction rate theory, with the energy barrier driven by a local deformation of the surface and a thermally activated relaxation.

Spacing-dependent dipolar interactions in dendronized magnetic iron oxide nanoparticle 2D arrays and powders

Self-assembly of nanoparticles (NPs) into tailored structures is a promising strategy for the production and design of materials with new functions. In this work, 2D arrays of iron oxide NPs with interparticle distances tuned by grafting fatty acids and dendritic molecules at the NPs surface have been obtained over large areas with high density using the Langmuir-Blodgett technique. The anchoring agent of molecules and the Janus structure of NPs are shown to be key parameters driving the deposition. Finally the influence of interparticle distance on the collective magnetic properties in powders and in monolayers is clearly demonstrated by DC and AC SQUID measurements. The blocking temperature T(B) increases as the interparticle distance decreases, which is consistent with the fact that dipolar interactions are responsible for this increase. Dipolar interactions are found to be stronger for particles assembled in thin films compared to powdered samples and may be described by using the Vogel Fulcher model.

Magnetism in gold nanoparticles

Gold nanoparticles currently elicit an intense and very broad research activity because of their peculiar properties. Be it in catalysis, optics, electronics, sensing or theranostics, new applications are found daily for these materials. Approximately a decade ago a report was published with magnetometry data showing that gold nanoparticles, most surprisingly, could also be magnetic, with features that the usual rules of magnetism were unable to explain. Many ensuing experimental papers confirmed this observation, although the reported magnetic behaviours showed a great variability, for unclear reasons. In this review, most of the experimental facts pertaining to "magnetic gold" are summarized. The various theories put forth for explaining this unexpected magnetism are presented and discussed. We show that despite much effort, a satisfying explanation is still lacking and that the field of hypotheses should perhaps be widened.

Liquid-crystalline nanoparticles: Hybrid design and mesophase structures

Liquid-crystalline nanoparticles represent an exciting class of new materials for a variety of potential applications. By combining supramolecular ordering with the fluid properties of the liquid-crystalline state, these materials offer the possibility to organise nanoparticles into addressable 2-D and 3-D arrangements exhibiting high processability and self-healing properties. Herein, we review the developments in the field of discrete thermotropic liquid-crystalline nanoparticle hybrids, with special emphasis on the relationship between the nanoparticle morphology and the nature of the organic ligand coating and their resulting phase behaviour. Mechanisms proposed to explain the supramolecular organisation of the mesogens within the liquid-crystalline phases are discussed.

The Quest for Nanoscale Magnets: The example of [Mn12] Single Molecule Magnets

Recent advances on the organization and characterization of [Mn12] single molecule magnets (SMMs) on a surface or in 3D are reviewed. By using nonconventional techniques such as X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM), it is shown that [Mn12]-based SMMs deposited on a surface lose their SMM behavior, even though the molecules seem to be structurally undamaged. A new approach is reported to get high-density information-storage devices, based on the 3D assembling of SMMs in a liquid crystalline phase. The 3D nanostructure exhibits the anisotropic character of the SMMs, thus opening the way to address micrometric volumes by two photon absorption using the pump-probe technique. We present recent developments such as µ-SQUID, magneto-optical Kerr effect (MOKE), or magneto-optical circular dichroism (MOCD), which enable the characterization of SMM nanostructures with exceptional sensitivity. Further, the spin-polarized version of the STM under ultrahigh vacuum is shown to be the key tool for addressing not only single molecule magnets, but also magnetic nano-objects.

Synthesis and physical properties of ferrocene derivatives. XXI. Crystal structure of a liquid crystalline ferrocene derivative, 1,1'-bis[3-[4-(4-methoxyphenoxycarbonyl)phenoxy]propyloxycarbonyl]ferrocene

The crystal structure of the title 1,1′-disubstituted ferrocene derivative was determined by X-ray diffraction using a single crystal. This compound exhibits a nematic phase only. The X-ray structure analysis revealed that the two substituents lie in the same direction (`U' shape) and the flexible spacer adopts agaucheconformation. These conformations are strongly related to the formation of a rod-like shape, which favors liquid crystalline behavior. In the crystal structure, C—H...π, π–π and T-stacking interactions were observed. It is considered that these interactions play a major role in stabilizing the molecular packing arrangement and the mesomorphism.

Magnetic Interactions in CuII−LnIII Cyclic Tetranuclear Complexes: Is It Possible to Explain the Occurrence of SMM Behavior in CuII−TbIII and CuII−DyIII Complexes?

An extensive series of tetranuclear CuII2LnIII2 complexes [CuIILLnIII(hfac)2]2 (with LnIII being all lanthanide(III) ions except for the radioactive PmIII) has been prepared in order to investigate the nature of the CuII-LnIII magnetic interactions and to try to answer the following question: What makes the CuII2TbIII2 and CuII2DyIII2 complexes single molecule magnets while the other complexes are not? All the complexes within this series possess a similar cyclic tetranuclear structure, in which the CuII and LnIII ions are arrayed alternately via bridges of ligand complex (CuIIL). Regular SQUID magnetometry measurements have been performed on the series. The temperature-dependent magnetic susceptibilities from 2 to 300 K and the field-dependent magnetizations from 0 to 5 T at 2 K have been measured for the CuII2LnIII2 and NiII2LnIII2 complexes, with the NiII2LnIII2 complex containing diamagnetic NiII ions being used as a reference for the evaluation of the CuII-LnIII magnetic interactions. These measurements have revealed that the interactions between CuII and LnIII ions are very weakly antiferromagnetic if Ln=Ce, Nd, Sm, Yb, ferromagnetic if Ln=Gd, Tb, Dy, Ho, Er, Tm, and negligible if Ln=La, Eu, Pr, Lu. With the same goal of better understanding the evolution of the intramolecular magnetic interactions, X-ray magnetic circular dichroism (XMCD) has also been measured on CuII2TbIII2, CuII2DyIII2, and NiII2TbIII2 complexes, either at the L- and M-edges of the metal ions or at the K-edge of the N and O atoms. Last, the CuII2TbIII2 complex exhibiting SMM behavior has received a closer examination of its low temperature magnetic properties down to 0.1 K. These particular measurements have revealed the unusual very slow setting-up of a 3D order below 0.6 K.

Interfacial behavior of a series of amphiphilic block co-dendrimers

Amphiphiles with a dendritic structure are attractive materials as they combine the features of dendrimers with the self-assembling properties and interfacial behavior of amphiphiles. We have designed and synthesized three series of segmented amphiphilic block co-dendrimers (Janus-type) and studied their interfacial properties on the Langmuir trough. Various behaviors are observed with, as a rule, the lowest generation dendrimers behaving more or less like traditional amphiphiles while the larger molecules tend to exhibit more complicated isotherms, with a non-straightforward temperature dependence, one particular molecule seemingly forming supramolecular assemblies spontaneously. The results presented here, obtained on a series of molecules where many parameters have been varied systematically, show the limits that should be kept in mind when designing amphiphilic dendrimers.

Self-assembly, DNA complexation, and pH response of amphiphilic dendrimers for gene transfection

Cationic lipids and polymers are routinely used for cell transfection, and a variety of structure-activity relation data have been collected. Few studies, however, focus on the structural aspects of self-assembly as a crucial control parameter for gene delivery. We present here the observations collected for a set of cationic dendritic amphiphiles based on a stiff tolane core (1-4) that are built from identical subunits but differ in the number and balance of their hydrophobic and cationic hydrophilic moieties. We established elsewhere that vectors 3 and 4 have promising transfection properties. Scanning probe microscopy (AFM, STM), cryo-transmission electron microscopy (cryo-TEM), and Langmuir techniques provide insight into the self-assembly properties of the molecules under physiological conditions. Furthermore, we present DNA and pH "jump" experiments where we study the response of Langmuir films to a sudden increase in DNA concentration or a drop in pH. We find that the primary self-assembly of the amphiphile is of paramount importance and influences DNA binding, serum sensitivity, and pH response of the vector system.

Effects of structural modification on gene transfection and self-assembling properties of amphiphilic dendrimers

A library of novel amphiphilic, self-assembling dendrimers was designed and synthesised to evaluate the effects of structural changes on transfection efficiency.

Preparation of highly stable organic steps with a fullerene-based molecule

We report the formation of highly stable Langmuir-Blodgett (LB) organic steps made with a hexa-adduct fullerene-based amphiphile. This amphiphile forms films of excellent quality, with a very low roughness, that are structurally stable: X-ray reflectivity spectra recorded on fresh and 12-month-old samples are undiscernible. Such a behavior contrasts with that of more traditional amphiphiles, which are unfortunately well-known for their instability in time. The stability of the films stems, among others, from the spheroidal shape of the constitutive molecules. These experiments show that it is possible to circumvent the major drawback of LB films and to prepare materials more suited for applications. We show that the LB film prepared with this fullerene derivative can successfully be used as thickness gauges for atomic force microscopy or light microscopy studies.

Amphiphilic Diblock Dendrimers with a Fullerene Core

Amphiphilic dendrimers with a C(60) core have been obtained by cyclization of dendritic 1,3-phenylenebis(methylene)-tethered bis-malonate derivatives at the carbon sphere. The relative position of the two cyclopropane rings in the resulting bis-methanofullerene derivatives has been determined based on the molecular symmetry (C(s)()) deduced from the (1)H and (13)C NMR spectra. The hydrophobic-hydrophilic balance of these dendrimers has been systematically modified by changing the size of the polar headgroup in order to investigate the role of the amphiphilicity both at the air-water interface and during deposition onto solid substrates. Langmuir studies have revealed a conformational change in the dendritic structure with the size of the polar headgroup. Because of a better anchoring onto the water surface, the compounds with the largest polar headgroup adopt a more compact structure and the dendritic branches are forced to wrap the fullerene core. This model is nicely confirmed by the amount of fullerene-fullerene interactions within the Langmuir-Blodgett films as deduced from their absorption spectra.

Amphiphilic diblock dendrimers: synthesis and incorporation in Langmuir and Langmuir-Blodgett films

A new dendron with peripheral long alkyl chains and containing five C(60) units in the branching shell has been prepared and attached to a Fréchet-type dendron functionalized with ethylene glycol chains. The peripheral substitution of the resulting globular dendrimer with hydrophobic chains on one hemisphere and hydrophilic groups on the other provides the perfect hydrophobic/hydrophilic balance allowing the formation of stable Langmuir films. Furthermore, a perfect reversibility has been observed in successive compression/decompression cycles. The diblock structure of the dendrimer has been also crucial for the efficient transfer of the Langmuir films in order to obtain well-ordered multilayered Langmuir-Blodgett films. This approach appears particularly interesting since functional groups not well adapted for the preparation of Langmuir and Langmuir-Blodgett films such as fullerenes can be attached into the branching shell of the dendritic structure and, thus, efficiently incorporated in thin ordered films.

Investigations of Thin Films with Amphiphilic Dendrimers Bearing Peripheral Fullerene Subunits

In spite of a molecular mass of 7704.6 g mol(-1), third-generation compound G3 (shown schematically; Z=C(8)H(17)) is able to form stable Langmuir films. In a systematic study, the amphiphilic properties of the corresponding dendrimers of first (G1) and second generation (G2), with one and two peripheral fullerene units, respectively, were investigated and a model could be proposed for the multilayer films obtained from G1.