Controlling the Pulsed-Laser-Induced Size Reduction of Au and Ag Nanoparticles via Changes in the External Pressure, Laser Intensity, and Excitation Wavelength

The laser-induced size reduction of aqueous noble metal nanoparticles has been the subject of intensive research, because of the mechanistic interest in the light–nanoparticle interactions and its potential application to size control. The photothermal evaporation hypothesis has gained solid support. However, the polydispersity of the final products is considered as an inherent drawback of the method. It is likely that the polydispersity arises from the uncontrolled heat dissipation caused by vapor bubble formation in the ambient atmosphere. To overcome this problem, we applied high pressures of 30–100 MPa. The particle size was regulated by adjusting three parameters: the pressure, laser intensity, and excitation wavelength. For example, starting from a colloidal solution of 100 nm diameter gold nanoparticles, highly monodisperse (±3–5%) spheres with various diameters ranging from 90 to 30 nm were fabricated by tuning the laser intensity at 100 MPa, using an excitation wavelength of 532 nm. Further size reduction of the diameter to 20 nm was achieved by reducing the pressure and switching the excitation wavelength to 355 nm. It was found that the application of high pressures led to the heat loss-controlled size-reduction of the gold nanoparticles. More complicated results were obtained for 100 nm silver nanoparticles, possibly because of the different size-dependent light-absorbing nature of these particles. Based on our extensive experimental studies, a detailed picture was developed for the nanosecond laser-induced fabrication of gold and silver nanoparticles, leading to unprecedented size control

Optical Scattering Spectral Thermometry and Refractometry of a Single Gold Nanoparticle under CW Laser Excitation

White light scattering spectra based on dark-field microscopy is a powerful tool for studying localized surface plasmon resonances of single noble metal nanoparticles and for developing their applications in sensing and biomedical therapies. Here we demonstrate the steady-state scattering spectral changes of a single gold nanoparticle under continuous laser heating. The experimental peak shifts allowed the estimation of the particle temperature in the range of 300–700 K with an accuracy of ±20 K on the basis of a spectral calculation exploiting Mie theory. For single gold nanoparticles supported on a glass substrate, progressive red shifts were observed in air, whereas blue shifts were observed in water and glycerol with increasing temperature. The medium has strong influence on peak shifts because of distance-dependent nanoscale medium heating: the shifts strongly depend on the magnitude of the temperature coefficients of a medium refractive index. The laser power-dependent behavior of peak shifts revealed the onset of surface melting occurring at 550–600 K regardless of the medium. Furthermore, experimental shifts also suggested that surface liquid layer grows in thickness with increasing temperature until the whole particle melts: this model has been proposed theoretically as a liquid nucleation and growth hypothesis. Therefore, we showed that the scattering spectral shifts represent an effective measure for the laser-induced morphological alterations of a gold nanoparticle and the nanoscale heating of a medium surrounding the NP

Observation of Nanoscale Cooling Effects by Substrates and the Surrounding Media for Single Gold Nanoparticles under CW-Laser Illumination

Understanding the nanoscale heating-induced local thermal response is important but hampered by lack of information on temperatures at such small scales. This paper reports laser-induced heating and thermal equilibration of metal nanoparticles supported on different substrates and immersed in several media. We use single-particle spectroscopy to monitor nanoparticle temperature rises due to laser excitation. Because of changes in the refractive index of the surrounding medium, the scattering spectrum of the gold nanoparticles undergoes a shift that is related to the temperature of the system. We find that the temperature increase depends on both the surrounding medium and the supporting substrate. We furthermore model the nanoparticle temperature using a simplified 1-D heat conduction model with an effective thermal conductivity that takes both substrate and environment into account. The results from this model are also compared to a more detailed 2-D heat transfer analysis. The results presented here are quite new and important to many plasmonic nanoparticle applications where the strong absorption cross section of the nanoparticles leads to a significant temperature rise. In particular, the current work introduces an analysis that can be easily implemented to model the temperature of a nanoparticle supported on a substrate, as is the case in many single-particle measurements

Reactivity of Bulky Formamidinatosamarium(II or III) Complexes with CO and CS Bonds

The preparation of a new heterobimetallic samarium­(II) formamidinate complex and selected reactions of samarium­(II) complexes and one samarium­(III) formamidinate complex with benzophenone or CS₂ are discussed. Treatment of the tris­(formamidinato)­samarium­(III) complex [Sm­(DippForm)₃] <b>1</b> (DippForm = <i>N</i>,<i>N</i>′-bis­(2,6-diisopropylphenyl)­formamidinate, (CH­(NC₆H₃-^<i>i</i>Pr₂-2,6)₂) with potassium graphite in toluene yielded the dark brown heterobimetallic formamidinatosamarium­(II)/potassium complex [KSm­(DippForm)₃]_<i>n</i> <b>2</b>. Divalent <b>2</b>, a Lewis base solvent free homoleptic species, differs significantly from the related heteroleptic formamidinatosamarium­(II) complex [Sm­(DippForm)₂(thf)₂] <b>3</b> with respect to its constitution, structure, and reactivity toward benzophenone. While <b>2</b> reacts giving complex <b>1</b>, the reaction of <b>3</b> with benzophenone generates the highly unusual [Sm­(DippForm)₂(thf)­{μ-OC­(Ph)(C₆H₅)­C­(Ph)₂O}­Sm­(DippForm)₂] (C₆H₅ = 1,4-cyclohexadiene-3-yl-6-ylidene) <b>4</b>. The formation of <b>4</b> highlights a rare C–C coupling between a carbonyl carbon and the carbon at the para position of a phenyl group of the OCPh₂ fragment. An analogous reaction of [Yb­(DippForm)₂(thf)₂] gives an isostructural complex <b>4Yb</b>. <b>3</b> reacts with carbon disulfide forming a light green dinuclear formamidinatosamarium­(III) complex [{Sm­(DippForm)₂(thf)}₂(μ-η²(C,S):κ­(S′,S″)-SCSCS₂)] <b>5</b> through an unusual C–S coupling induced by an amidinatolanthanoid species giving the thioformylcarbonotrithioate ligand. The trivalent organometallic [Sm­(DippForm)₂(CCPh)­(thf)] complex activates the CO bond of benzophenone by an insertion reaction, forming the light yellow [Sm­(DippForm)₂{OC­(Ph)₂C₂Ph}­(thf)] <b>6</b> as a major product and light yellow unsolvated [Sm­(DippForm)₂{OC­(Ph)₂C₂Ph}] <b>7</b> as a minor product. Molecular structures of complexes (<b>2</b>, <b>4</b>–<b>7</b>) show that κ­(<i>N</i>,<i>N</i>′) bonding between a DippForm and samarium atom exists in all compounds, but in <b>2</b>, DippForm also bridges K and Sm by 1κ­(N):2κ­(N′) bonding and two 2,6-diisopropylphenyl groups are η⁶-bonded to potassium

Picosecond-to-Nanosecond Dynamics of Plasmonic Nanobubbles from Pump–Probe Spectral Measurements of Aqueous Colloidal Gold Nanoparticles

The photothermal generation of nanoscale vapor bubbles around noble metal nanoparticles is of significant interest, not only in understanding the underlying mechanisms responsible for photothermal effects, but also to optimize photothermal effects in applications such as photothermal cancer therapies. Here, we describe the dynamics in the 400–900 nm regime of the formation and evolution of nanobubbles around colloidal gold nanoparticles using picosecond pump–probe optical measurements. From excitations of 20–150 nm colloidal gold nanoparticles with a 355 nm, 15 ps laser, time-dependent optical extinction signals corresponding to nanobubble formation were recorded. The extinction spectra associated with nanobubbles of different diameters were simulated by considering a concentric spherical core–shell model within the Mie theory framework. In the simulations, we assumed an increase in particle temperature. From temporal changes in the experimental data of transient extinctions, we estimated the temporal evolution of the nanobubble diameter. Corrections to bubble-free temperature effects on the transient extinction decays were applied in these experiments by suppressing bubble formation using pressures as high as 60 MPa. The results of this study suggest that the nanobubbles generated around a 60 nm-diameter gold nanoparticle using a fluence of 5.2 mJ cm^–2 had a maximum diameter of 260 ± 40 nm, and a lifetime of approximately 10 ns. The combination of fast transient extinction spectral measurements and spectral simulations provides insights into plasmonic nanobubble dynamics

Titanium(IV) Surface Complexes Bearing Chelating Catecholato Ligands for Enhanced Band-Gap Reduction

Protonolysis reactions between dimethylamido titanium(IV) catecholate [Ti(CAT)(NMe2)2]2 and neopentanol or tris(tert-butoxy)silanol gave catecholato-bridged dimers [(Ti(CAT)(OCH2tBu)2)(HNMe2)]2 and [Ti(CAT){OSi(OtBu)3}2(HNMe2)2]2, respectively. Analogous reactions using the dimeric dimethylamido titanium(IV) (3,6-di-tert-butyl)catecholate [Ti(CATtBu2-3,6)(NMe2)2]2 yielded the monomeric Ti(CATtBu2-3,6)(OCH2tBu)2(HNMe2)2 and Ti(CATtBu2-3,6)[OSi(OtBu)3]2(HNMe2)2. The neopentoxide complex Ti(CATtBu2-3,6)(OCH2tBu)2(HNMe2)2 engaged in further protonolysis reactions with Si–OH groups and was consequentially used for grafting onto mesoporous silica KIT-6. Upon immobilization, the surface complex [Ti(CATtBu2-3,6)(OCH2tBu)2(HNMe2)2]@[KIT-6] retained the bidentate chelating geometry of the catecholato ligand. This convergent grafting strategy was compared with a sequential and an aqueous approach, which gave either a mixture of bidentate chelating species with a bipodally anchored Ti(IV) center along with other physisorbed surface species or not clearly identifiable surface species. Extension of the convergent and aqueous approaches to anatase mesoporous titania (m-TiO2) enabled optical and electronic investigations of the corresponding surface species, revealing that the band-gap reduction is more pronounced for the bidentate chelating species (convergent approach) than for that obtained via the aqueous approach. The applied methods include X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and solid-state UV/vis spectroscopy. The energy-level alignment for the surface species from the aqueous approach, calculated from experimental data, accounts for the well-known type II excitation mechanism, whereas the findings indicate a distinct excitation mechanism for the bidentate chelating surface species of the material [Ti(CATtBu2-3,6)(OCH2tBu)2(HNMe2)2]@[m-TiO2]