David J. Elliot

Formation of CdS and HgS nanoparticles in LB films

Abstract Langmuir–Blodgett (LB) films were constructed on hydrophobic quartz substrates from monolayers of behenic acid on subphases containing only Cd 2+ or Hg 2+ and containing a mixture of Cd 2+ and Hg 2+ ions. The reaction of these films with H 2 S to form metal sulphide particles was investigated using UV/visible spectroscopy and X-ray photoelectron spectroscopy (XPS). For the mixed metal ion system it was concluded that reaction with H 2 S results in individual CdS and HgS particles forming rather than a mixed cadmium mercury sulphide. In order to investigate the kinetics of the H 2 S reaction, the frequencies of MBe (M=Cd or Hg) coated quartz crystal microbalances were followed as a function of H 2 S exposure time. Immersion of CdS particles into a Hg 2+ solution resulted in a change in the UV–visible absorbance spectrum attributed to the exchange of Cd 2+ ions in the particles for Hg 2+ ions most likely resulting in a HgS layer around the CdS core.

Preparation of Q-state particles in Langmuir–Blodgett films

Abstract This review details the methods used in the production of a range of Q-state semiconductor and metal particles in Langmuir–Blodgett films, and describes some of the many methods used to characterise the particle-films. Chalcogenide based semiconductor particles can be grown in Langmuir–Blodgett films by cycles of immersion in a solution containing the appropriate metal cation and exposure to a chalcogenide gas. The preparation of metal chalcogenide (MX, where X=O, S, Se, Te) particles is discussed in terms of mechanistic insights into particle formation and growth. In particular, research investigating the role of the metal ion coordination sphere in the rate of particle formation and the roles of water and chemical capping in particle growth are presented. Various analytical techniques have been used to characterise the films including UV/visible absorption and grazing angle FTIR spectroscopies, atomic force microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, surface plasmon resonance and quartz crystal microbalance gravimetry. Methods for manipulating the optical properties of the particle-films, such as the production of core-shell particles and co-existent particles of different composition are presented. The formation of Q-state metal particles by CO or hydrazine reduction is referred to as an analogous technique. Finally, the results of photoelectrochemical and photobleaching measurements, are discussed in light of their potential usage as components in photoactivated devices.

In-situ measurement of lead(II) ion binding to an arachidic acid langmuir monolayer using a quartz crystal microbalance

Abstract A quartz crystal microbalance (QCM) was used to measure the in situ binding of lead (II) ions to a Langmuir monolayer of arachidic acid (ArH) at the air-water interface. A horizontally oriented QCM was lowered onto an ArH monolayer and an analiquot of a Pb 2+ solution was injected beneath the monolayer. A decrease in the QCM frequency was observed, consistent with Pb 2+ binding to the ArH monolayer. Models for a fatty acid to divalent metal ion ratio ( F : M ) of 2 and 1 were considered. The experimental data was consistent with 2:1 binding.

Preparation of HgS in Langmuir-Blodgett films of mercury(II) arachidate and mercury(II) behenate

Abstract Langmuir-Blodgett (LB) films of mercury(II) arachidate (HgAr) and mercury(II) behenate (HgBe) were deposited on hydrophobed quartz and mica using airwater monolayers of the respective fatty acids spread on subphases containing Hg(OOCCH 3 ) 2 . The reactions of HgAr and HgBe films with H 2 S were investigated using ultraviolet-visible-near infrared spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy (AFM), and quartz crystal microbalance techniques. It was concluded that HgS forms quantitatively in the LB films. Additional HgS can be introduced into the film after gassing by immersion of the films in a Hg 2+ solution and regassing with H 2 S. Absorbance data do not provide information as to the nature and size of the HgS aggregates in the film. AFM measurements, however, indicate that individual particles are formed in the size range 2–10 nm. Agglomerates of individual particles were also observed.

Reactions of complex ions of platinum and palladium in Langmuir-Blodgett films of dimethyldioctadecylammonium chlorometallates

Abstract Langmuir-Blodgett films containing complex ions of platinum(II), platinum(IV) and palladium(II) were prepared from compressed monolayers of dimethyldioctadecylammonium bromide on subphases of these ions. The films were characterized by measuring the surface pressure vs. area per molecule ( π A ) isotherms and by UV/visible spectroscopy, grazing angle FTIR spectroscopy and X-ray photoelectron spectroscopy. The reactions of these films with H2S and hydrazine were investigated by the above spectroscopic techniques and by atomic force microscopy (AFM). The absorbance onsets of the H2S-exposed films are dramatically blue shifted relative to the bulk bandgaps of PtS2, PtS and PdS. This is consistent with the formation of particles in the “Q state” size regime. AFM images of hydrazine-treated (DDA)2(PdCl4) films on mica revealed metal particles with an average diameter of 3.6 nm.

Ultrafast dynamics of fluorescence-activated CdS nanoparticles in aqueous solutions by femtosecond transient bleaching spectroscopy

Abstract The ultrafast dynamics of transient photobleaching and recovery of fluorescence-activated cadmium sulphide (CdS) nanoparticles in aqueous solutions upon treatment with excess cadmium ions have been studied upon excitation at 400 nm with a femtosecond laser system. Three different sized nanoparticles were prepared with a diameter of 5.7, 4.4 and 3.7 nm. The colloids showed strong fluorescence bands at 504, 477 and 458 nm, respectively, which were close to the exciton absorption. Transient photobleaching of less than 1 ps with a peak quite similar to the wavelength of the exciton absorption band was observed for the two larger samples. It was found that the bleaching component with a lifetime of several picoseconds became pronounced upon treatment with excess Cd2+. A much faster component with a subpicosecond lifetime was also observed. These results strongly suggest that the direct recombination of charge carriers dominates in fluorescence-activated CdS nanoparticles.

Preparation and spectral characteristics of gold particles in Langmuir-Blodgett films

Langmuir-Blodgett films incorporating complex ions of gold(III) have been fabricated from air-water monolayers of arachidic acid, dimethyldioctadecylammonium bromide and dihexadecyl phosphate. The films, and their reactions with reducing agents to produce colloidal gold particles, have been characterised by surface pressure/area per molecule (π-A) isotherms, UV-vis. and XPS spectroscopy, atomic force microscopy (AFM) and transmission electron microscopy (TEM). Colloidal gold particles, produced in films exposed to carbon monoxide, were examined by AFM and TEM. The gold particles, as determined by TEM, were of a mean size of 10 nm and were randomly distributed in the films. Solvent immersion studies were also conducted in order to investigate the effect of the dielectric medium on the position of the gold colloid plasmon band. The position of the wavelength maximum of the gold plasmon band of the gold-LB film in air was not consistent with a just a layer of surfactant surrounding the particles. From the position of the surface plasmon band peak an effective refractive index of 1.35 is calculated for the LB film, which is considerably lower than that of a dense surfactant film. Also, the band maximum did not shift to any measurable extent when the films were immersed in water. We suggest that the reason for these observations is that air is trapped in the film and this lowers the refractive index of the film and prevents water penetration into the film. Immersion of a gold-LB film in the high refractive index liquid CS2 resulted in a redshift in the gold plasmon band, which was a consequence of the aggregation of the gold particles due to the dissolution of the surfactant film in the surrounding liquid.

Ultrafast transient bleaching of CdS nanoparticles in various media

Ultrafast dynamics of transient photobleaching and recovery of cadmium sulphide (CdS) nanoparticles of different size ranging from less than 3.5 nm to greater than 5.0 nm, dispersed in Nafion films, in self-assembled films, and in aqueous solutions were studied upon excitation at 400 nm with a femtosecond laser. All samples gave transient bleaching in less than 1 ps and a multi-exponential recovery in the wavelength region above about 410 nm. The peak wavelength of the transient bleaching and its time dependence together with the recovery behavior were affected by the particle size, surface treatment and the supporting medium. The peak of the transient bleaching spectrum of larger CdS nanoparticles showed a red-shift during the recovery process, while for the smaller particles no such change was observed. CdS nanoparticles in self-assembled films showed an extremely long-lived bleaching recovery process. Fluorescence-activated CdS nanoparticles in aqueous solutions, which showed very strong and sharp fluorescence near the exciton absorption, showed very fast recovery. These results were explained by the decay of excitons and the recombination of surface trapped electron-hole pairs with different energies depending on the particle size.

The Formation and Properties of Nano-Sized Semiconductor Particles in Langmuir-Blodgett Films

The insitu formation of Q-state semiconductor particles in Langmuir-Blodgett (LB) films is but one of many ways these particles can be synthesised. LB films not only act as a matrix that limits the size of the particles that are formed, but also act as a “vehicle” by which these particles can be deposited onto substrates of interest. LB films themselves may contain molecules of specific functional value that may complement certain aspects of the inorganic particles imbedded in the layered structure. In addition, LB films can be constructed consisting of as little as one monolayer, very nearly the lower limit of miniaturisation. This is a very attractive feature considering the commercial markets available for ultra small devices.