Interparticle Coupling Research Articles

Response of magnetic nanoparticle assemblies

Properties of magnetic nanoparticle assemblies are analysed by employing the randomly jumping interacting moments model including quantum fluctuations, ferromagnetic inter-particle coupling and disorder. Using spatially uniform external field we find the respective magnetic state equation and phase diagram indicating an existence of spinodal regions and critical points. Possibilities to use such systems as magnetoresistive sensors are considered.

Entanglement resonances of driven multi-partite quantum systems

We show how to create maximally entangled dressed states of a weakly interacting multi-partite quantum system by suitably tuning an external, periodic driving field. Floquet theory allows us to relate, in a transparent manner, the occurrence of entanglement resonances to avoided crossings in the spectrum of quasi-energies, tantamount of well-defined conditions for the controlled, resonant interaction of particles. We demonstrate the universality of the phenomenon for periodically driven, weakly interacting two-level systems, by considering different interaction mechanisms and driving profiles. In particular, we show that entanglement resonances are a generic feature of driven, multi-partite systems, widely independent of the details of the interaction mechanism. Our results are therefore particularly relevant for experiments on interacting two-level systems, in which the microscopic realization of the inter-particle coupling is unknown.

Interparticle coupling of plasmon fields due to reorganization of Au nanoparticles in Langmuir-Blodgett film

Dodecanethiol-encapsulated Au nanoparticles have been deposited on Si(001) and quartz substrates by Langmuir-Blodgett method at different surface pressures for producing layer-by-layer structure. Out-of-plane structural reorganization and optical absorption spectra have been obtained from x-ray reflectivity and UV-Vis spectroscopy measurements, respectively. With time, positions of the nanoparticles reorganize so that films become more compact and thickness decreases. Localized surface plasmon resonance peaks show that coupling between Au nanoparticles occur as the interparticle distance decreases after reorganization and as a result, a redshift in the plasmon peak wavelength takes place. Variation in redshift depends upon the deposition pressure or layer number.

Hierarchically Ordered Arrays of Noncircular Silicon Nanowires Featured by Holographic Lithography Toward a High‐Fidelity Sensing Platform

AbstractA novel, highly uniform and tunable hybrid plasmonic array is created via ion‐milling, catalytic wet‐etching and electron‐beam evaporation, using a holographically featured structure as a milling mask. A simple and low‐cost prism holographic lithography (HL) technique is applied to create an unprecedentedly coordinated array of elliptic gold (Au) holes, which act as the silicon (Si) etching catalyst in the reaction solution used to fabricate an elliptic silicon nanowire (SiNW) array; here, the SiNWs are arrayed hierarchically in such a way that three SiNWs are triangularly coordinated, and the triangles are arranged hexagonally. After removing the polymeric mask and metal thin film, the highly anisotropic thick Au film is deposited on the SiNW arrays. This hybrid substrate shows tunable optical properties in the near‐infrared (NIR) region from 875 nm to 1030 nm and surface‐enhanced Raman scattering (SERS) activities; these characteristics depend on the catalytic wet etching time, which changes the size of the vertical gap between the Au thick films deposited separately on the SiNWs. In addition, lateral interparticle coupling induces highly intensified SERS signals with good homogeneity. Finally, the Au‐capped elliptical SiNW arrays can be hierarchically patterned by combining prism HL and conventional photolithography, and the highly enhanced fluorescence intensity associated with both the structural effects and the plasmon resonances is investigated.

Solid Phase Supported “Click”-Chemistry Approach for the Preparation of Water Soluble Gold Nanoparticle Dimers

We investigated the potential of the Cu(I) catalyzed azide-alkyne cycloaddition between water soluble azide and alkyne functionalized gold nanoparticles in terms of dimer formation via a solid phase support. Alkyne and azide lipoic acid derivatives are prepared and utilized as stabilizing ligands for 15 nm gold colloids. For the solid phase supported click reaction first citrate stabilized gold nanoparticles are immobilized on amine terminated silicon wafers. In the following step the citrate ligands of the upper free accessible nanoparticle surface are exchanged against a mixture of the alkyne derivative of lipoic acid and lipoic acid. Upon addition of lipoic acid/lipoic acid azide derivative stabilized 15 nm gold nanoparticles and the Cu(I) catalyst solution covalent interparticle coupling between immobilized and gold nanoparticles added is achieved. The formed structures are analyzed by scanning electron microscopy directly on the solid support. It is demonstrated that the yield of dimeric structures on the solid phase support increases with increased molar ratio of the catalyst, thus indicating that dimers are indeed formed by covalent bond formation. Upon treatment with ultrasound the formed structures could be released and detected with transmission electron microscopy measurements.

Site-Selective Assembly and Reorganization of Gold Nanoparticles along Aminosilane-Covered Nanolines Prepared on Indium–Tin Oxide

We have fabricated gold nanoparticle (AuNP) arrays on indium-tin oxide (ITO) substrates in a nearly one-dimensional fashion. AuNPs were site-selectively immobilized on ITO of which the surface had been patterned by a nanolithography process based on scanning probe microscopy. The fabricated nanoscale lines covered with aminosilane self-assembled monolayer served as chemisorption sites for citrate-stabilized AuNPs of 20 nm in diameter, accordingly, AuNP nanolines with a thickness of single nanoparticle diameter were spontaneously assembled on the lines. In this 1D array, the AuNPs were almost separated from each other due to the electrostatic repulsion between their negatively charged surface layers. Furthermore, a reorganization process of the immobilized AuNP arrays has been successfully demonstrated by replacing each AuNP's surface layer from citric acid to dodecanethiol. By this process, the AuNPs lost their electrostatic repulsion and became hydrophobic so as to be attracted to each other through hydrophobic interaction, resulting in reorganization of the AuNP array. By repeating the deposition and reorganization cycle, AuNPs were more densely packed. The optical absorption peak of the arrays due to their plasmonic resonance was found to shift from 526 to 590 nm in wavelength with repeating cycles, indicating that the resonance manner was changed from the single nanoparticle mode to the multiple particle mode with interparticle coupling.

Size and period optimization of front patterned interacting metal nanoparticles for maximizing absorption of solar radiation in amorphous silicon thin films

Plasmonic assisted enhancement of optical absorption within a thin layer of amorphous silicon (a-Si:H) by incorporating a periodic array of spherical metal nanoparticles on the front side of the a-Si:H layer has been demonstrated by solving Maxwell’s curl equations using a finite difference time domain (FDTD) method. For different particle sizes between 40 and 80 nm, the interparticle spacing has been optimized to yield maximum absorption of AM1.5G solar radiation. By increasing the interparticle coupling, a red-shift up to 280 nm is observed in the peak of broadband absorption behavior within a-Si:H layer. Meanwhile, a comparison between spherical Ag, Al and Au nanoparticles of fixed size is made with respect to the optical absorption enhancement within a-Si:H for the wavelength range from 400 to 800 nm from which we can choose the wavelength range over which a particular type of metal nanoparticle will be useful.

Magnetization Reversal of Rectangular Particles: Closure States and Effect of Dipolar Coupling

We have investigated the magnetization configurations of ferromagnetic rectangular particles with lateral dimensions 1025 × 450 nm2 and two different thicknesses: 10 and 40 nm. For each thickness, we analyzed both an array consisting of isolated particles and a second one where the nanomagnets are put head-to-tail, with 85 nm interdot spacing, forming long chains of closely-spaced elements. Magneto-optical Kerr effect measurements and in-field magnetic force microscopy experiments were performed to achieve a deep comprehension of the magnetization reversal process. It is shown that inter-particle dipolar coupling substantially modifies the reversal mechanism in the case of the largest thickness, where it can delay or even suppress the occurrence of closure configurations of the magnetization (with one or more vortices), that are typical of isolated nanomagnets.

Comparing the interparticle coupling effect on sensitivities of silver and gold nanoparticles

The wavelength shift of surface plasmon resonance peak resulting from the electromagnetic coupling noble metal nanoparticle increases with the increase in the dielectric constant of the medium and the decrease in the interparticle separation distance. In this work, the discrete dipole approximation method was used to calculate the extinction efficiency spectra of the silver–silver and gold–gold nanoparticle pairs. This work shows that the silver coupled-particle system has higher plasmon resonance sensitivity as compared to the gold coupled-particle system. However, the silver coupled-particle system has lower and a faster near-exponential decay of sensitivity enhancement factor than the gold coupled-particle system. Thus, the silver coupled-particle may be more suited for sensing applications as compared to the gold coupled-particle, but the interparticle coupling effect displays more pronounced effect on the gold coupled-particle system as compared to the silver coupled-particle system.

In-situ growth of silver nanostructure on quartz glass substrates

Silver nanostructure compact aggregates on the surface of quartz glass substrates were synthesized by small gold seeds with the assistance of poly vinypyrrolidone (PVP) and irradiation of fluorescent lamp. The formation mechanism of silver nanostructure was proposed. The results show that both the PVP and the light irradiation are the keys to in-situ growth of silver nanostructure on quartz glass substrates. The silver nanostructure of the substrates which finally grow up to 150 nm after 20 h irradiation exhibits irregular shape, and some of nanoparticles stack to form bilayer. A new broad band appears in the absorption spectra of the substrates due to the interparticle dipole-dipole coupling of surface plasmon resonance response of the triangular silver nanoplate particles, which red shifts 600–800 nm as the particles grow up. The substrates have an emission band centered at 400 nm on their fluorescence spectra, and the fluorescence intensity shrinks as the average size of the silver nanostructure increases. The strongest SERS signal of SERS-active substrate is fabricated after 16 h.

Plasmonic coupled nanotorch structures leading to uniform surface enhanced Raman scattering detection

Nanocrescent-like structures have become important surface enhanced Raman scattering structures because instead of relying on a few nanometer gap inter-particle plasmonic coupling to achieve local field enhancement, intra-particle plasmonic coupling between the cavity modes and the tip edges are utilized to achieve high local field enhancement at the tips. Our fabrication approach creates 'nanotorch' structures with controllable cavity rim opening and deterministic orientation to yield uniform Raman measurements, consisting of three-dimensional upright-oriented nanocrescent structures resting on nanopillars. Each structure serves as a single SERS substrate. We demonstrate that a nanotorch with a smaller rim opening results in a higher enhancement factor compare to one with a larger opening. More importantly, the uniformity of all the analysed Raman modes of adsorbed benzenethiol is better than 80% due to the consistent, upright orientation of each single nanotorch SERS substrate, paving the way for practical implementation of SERS detection.

Simple and green synthesis of monodisperse silver nanoparticles and surface-enhanced Raman scattering activity

We describe a simple and green method for preparing silver nanoparticles with an average diameter in the range from 4.0 nm to 5.0 nm and with the standard deviation as low as 0.3 nm (polydispersity: 5.5%). The pre-synthesized silver oleate precursor in oleic acid was thermally reduced to obtain the monodisperse silver nanoparticles without a size selection procedure. This environmentally friendly chemistry approach requires only three reagents (silver nitrate, sodium oleate and oleic acid) which all come from renewable resources. The average diameter of these nanoparticles can be greatly adjusted by varying reaction time and finely regulated by varying reaction temperature at the same precursor concentration. The nucleation/diffusional growth model can be used to interpret the size and morphology difference of silver nanoparticles synthesized under different conditions. Raman spectra of graphene with and without an active layer of silver nanoparticles were measured to investigate the surface-enhanced Raman scattering properties of the silver nanoparticles. The Raman signal of graphene with an active layer increased by fifteen-fold in comparison to that without an active layer. The surface-enhanced Raman scattering activity can be explained using interparticle coupling induced Raman enhancement.

Ligand Exchange on Colloidal CdSe Nanocrystals Using Thermally Labile tert-Butylthiol for Improved Photocurrent in Nanocrystal Films

As-prepared CdSe nanocrystals were ligand exchanged using tert-butylthiol, which yielded stable CdSe nanocrystal inks in the strong donor solvent tetramethylurea. The efficacy of ligand exchange was probed by thermogravimetric analysis (TGA) and FT-IR spectroscopy. By studying sequential exchanges of tetradecylphosphonic acid and then tert-butylthiol, TGA and energy dispersive X-ray spectroscopic evidence clearly demonstrated that the ligand exchange is essentially quantitative. The resulting tert-butylthiol-exchanged CdSe nanocrystals undergo facile thermal ligand expulsion (≤200 °C), which was studied by TGA-mass spectrometry. Mild thermal treatment of tert-butylthiol-exchanged CdSe nanocrystal films was found to induce loss of quantum confinement (as evidenced by UV-vis spectroscopy) and provided for increased electrochemical photocurrent, electron mobility, and film stability. Pyridine-exchanged CdSe nanocrystals were employed as a control system throughout to demonstrate the beneficial attributes of tert-butylthiol exchange; namely, lower organic content, better colloidal stability, improved interparticle coupling, and vastly increased electrochemical photocurrent response upon illumination.

Scrutinising magnetic disorder through metastable 3d- and 4f-nanostructured alloys

Mechanical milling of magnetic alloys 3d-(Fe) and 4f-based (Tb) results in the promotion of magnetic disorder. In Fe22Cu14Ag64 the nanoscopic arrangement has been revealed by a very comprehensive multipattern Rietveld analysis using a combination of four sets of data including X-ray and neutron diffraction. The nanostructure is formed by Fe61Cu39 particles of D=4.6nm with a collinear ferromagnetic structure in a metallic matrix constituted by D=11.9nm Ag nanoparticles. The creation of an ensemble of nanoparticles is favoured by the immiscible character of the starting metals. The nanostructured alloy presents a magnetic transition at around 160K which cannot be considered as a pure single-domain blocking but affected by interparticle dipolar interactions. The latter is confirmed by the spin dynamics, displaying a critical slowing down of the AC-susceptibility and a definite peak of the non-linear susceptibility. When the matrix is alternatively formed by Au nanoparticles in the Fe14Au86 (D=77nm) and Fe10Cu10Au80 (D=35nm) alloys, the magnetic response resembles that of a reentrant state as the milling time is not enough to reduce the particle size, triggering ferromagnetic interparticle coupling enhanced by a multidomain magnetic structure. In milled 4f-alloys formed by miscible Tb and Al, as TbAl2, the production process promotes the magnetic disorder modifying the pure ferromagnetic order of the bulk alloy and simultaneously the existence of particles (D=20nm) which are affected by the structural disorder.

ChemInform Abstract: Construction of Heterostructure Materials Toward Functionality

AbstractReview: 91 refs.

Synthesis, density functional theory, molecular dynamics and electrochemical studies of 3-thiopheneacetic acid-capped gold nanoparticles

Abstract Gold nanoparticles capped with a bifunctional ligand, 3-thiopheneacetic acid (3-TAA) were synthesised by borohydride reduction at room temperature. The transmission electron microscopy (TEM) analysis showed that the particle aggregates and had semi-linear partial linkages that could be attributed to multi-modal binding of the ligand with various gold nanoparticles through the terminal thiolether (–S–) group and oxygen of the carboxylic (–COOH) group. This bimodal interaction led to limited stability of the resultant nanoparticles when tested using highly electrolytic media. To investigate further, density functional theory (DFT) quantum chemical and molecular dynamic calculations were conducted. The energetically favorable binding modes of the ligand to gold nanoparticle surfaces using the Gaussian program were studied. The DFT results showed kinetic stability of Au–3-TAA–Au interactions leading to inter-particle coupling or aggregation. Electrochemical analysis of the resultant nature of the capping agent revealed that 3-thiopheneacetic acid did not form a polymer during the preparation of Au–3-TAA. The cyclic voltammograms of Au–3-TAA nanoparticles coated glassy carbon electrode showed a typical gold character with the oxidation and reduction peaks at 1.4 V and 0.9 V, respectively.

Influence of confinement on single-electron charging in a network of nanoparticles

We investigated the single-electron tunneling (SET) behavior in a network of ligand stabilized Au nanoparticles (NPs) that are self-organized on an Au(111) surface by means of low-temperature scanning tunneling microscopy and spectroscopy. We demonstrate that for a proper combination of ligand chain length and NP radius the ligand shell is able to isolate a particle from the neighboring ones. This results in SET spectra with a clear Coulomb blockade and a regular staircase, similar to SET spectra obtained for isolated particles. A fraction of the investigated particles exhibits additional fine structure on top of the Coulomb charging peaks in the tunneling conductance spectra. The origin of the fine structure can be related to quantum size effects due to the very small NP size rather than to inter-particle capacitive coupling. Our findings indicate the possibility of using an individual particle in the self-organized network as the central Coulomb island in a double-barrier tunnel junction configuration, similar to the case of an isolated particle.

Spin-glasslike behavior of magnetic ordered state originating from strong interparticle magnetostatic interaction in α-Fe nanoparticle agglomerate

In strongly interacting nanoparticle agglomerate through dipolar coupling, prepared using chemically synthesized α-Fe nanoparticle, the temperature Tpeak at which the zero-field-cooled magnetization exhibits its peak reaches 385 K. This indicates that the magnetic ordering appears above room temperature. Aging phenomena and memory effect observed below Tpeak are intrinsically same as that of spin glasses. The appearance of spin-glasslike behavior near room temperature should be attributed to the three-dimensional interparticle coupling that is purely magnetostatic.

Self‐Assembled Large Au Nanoparticle Arrays with Regular Hot Spots for SERS

The cost-effective self-assembly of 80 nm Au nanoparticles (NPs) into large-domain, hexagonally close-packed arrays for high-sensitivity and high-fidelity surface-enhanced Raman spectroscopy (SERS) is demonstrated. These arrays exhibit specific optical resonances due to strong interparticle coupling, which are well reproduced by finite-difference time-domain (FDTD) simulations. The gaps between NPs form a regular lattice of hot spots that enable a large amplification of both photoluminescence and Raman signals. At smaller wavelengths the hot spots are extended away from the minimum-gap positions, which allows SERS of larger analytes that do not fit into small gaps. Using CdSe quantum dots (QDs) a 3-5 times larger photoluminescence enhancement than previously reported is experimentally demonstrated and an unambiguous estimate of the electromagnetic SERS enhancement factor of ≈10(4) is obtained by direct scanning electron microscopy imaging of QDs responsible for the Raman signal. Much stronger enhancement of ≈10(8) is obtained at larger wavelengths for benzenethiol molecules penetrating the NP gaps.

Optical Impedance Matching Using Coupled Plasmonic Nanoparticle Arrays

Silver nanoparticle arrays placed on top of a high-refractive index substrate enhance the coupling of light into the substrate over a broad spectral range. We perform a systematic numerical and experimental study of the light incoupling by arrays of Ag nanoparticle arrays in order to achieve the best impedance matching between light propagating in air and in the substrate. We identify the parameters that determine the incoupling efficiency, including the effect of Fano resonances in the scattering, interparticle coupling, as well as resonance shifts due to variations in the near-field coupling to the substrate and spacer layer. The optimal configuration studied is a square array of 200 nm wide, 125 nm high spheroidal Ag particles, at a pitch of 450 nm on a 50 nm thick Si(3)N(4) spacer layer on a Si substrate. When integrated over the AM1.5 solar spectral range from 300 to 1100 nm, this particle array shows 50% enhanced incoupling compared to a bare Si wafer, 8% higher than a standard interference antireflection coating. Experimental data show that the enhancement occurs mostly in the spectral range near the Si band gap. This study opens new perspectives for antireflection coating applications in optical devices and for light management in Si solar cells.