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Detection and Characterization of Individual Micro- and Nanoparticles

Final Report Summary - DECIMA (Detection and Characterization of Individual Micro- and Nanoparticles)

The Project aimed to develop novel approaches for detection and characterization of particles in the critical nanometer – micrometer size range. The Project objectives include (a) the development of a method for the depth-resolution analysis of micro- and nanoparticles using femtosecond laser ablation mass spectrometry and (b) the demonstration of the sensitive detection of neutral nanoparticles using a graphene mass sensor in combination with particle elemental analysis using fs-laser ablation mass spectrometry. The work performed during the period of the Project implementation involved:
(a) development and setting up a new apparatus for mass spectrometric detection and characterization of individual nanoparticles using the ultrashort pulsed laser ablation technique, (b) carrying out test experiments of plasma plumes produced by laser ablation of bulk materials to test and optimize the characteristics of the mass spectrometric apparatus,
(c) development of a method based on the laser-induced forward transfer (LIFT) technique for delivering nanoparticles into the gas phase for analysis,
(d) performing experiments on blister-based LIFT of well characterized core-shell nanoparticles from a metal-coated glass substrate for mass spectrometric analysis under high-vacuum conditions,
(e) analysis of the LIFT process using atomic force microscopy (AFM) studies and theoretical considerations, and
(f) carrying out experiments on LIFT of fullerene molecules C60 for gas-phase analysis and studying the C60 transfer mechanisms.

The main results achieved during the Project implementation are:
(1) A new apparatus for gas-phase analysis of individual micro- and nanoparticles has been developed involving (a) time-of-flight mass spectrometry of both positive and negative ions, (b) laser-induced forward transfer (LIFT) of neutral particles into the ionization region of the mass spectrometer using a ns laser, and (c) ionization/excitation of the particles in the gas phase using a fs laser.
(2) A new effect of delayed emission of ions from the laser plasma produced in an external electric field has been revealed. It has been demonstrated that this effect can be used to control the cluster formation process in the laser ablation plumes.
(3) A new method for uniform deposition of nanoparticles from a water solution on strongly hydrophobic metal surfaces has been developed. In this method, a droplet of solution is squeezed between two surfaces, metal and glass, to produce a uniform aqueous layer and then water is evaporated by heating.
(4) A recently proposed matrix-free blister-based LIFT technique has been modified for introducing nanoparticles into high vacuum for analysis. The feasibility of this method has been demonstrated with well characterized gold-coated silica nanoparticles using the developed time-of-flight apparatus. The irradiation conditions for efficient particle emission from a titanium-coated glass substrate due to transient blister formation have been determined. In a deep fs-laser ablation regime, all constituents of the particles have been observed in the mass spectrum (as Si, SiO and Au species). The particle transfer efficiency has been achieved as high as ~90% in the optimal LIFT regimes. The average velocity of the transferred nanoparticles has been measured to be around 50 m/s. The angular distributions of the LIFT-ejected particles have been investigated with a specially designed receiver system and found to be very narrow (particle beam divergence is around 5 degrees).
(5) An insight into the blister-based LIFT process has been obtained using AFM studies and theoretical considerations. Correlations between AFM images of the laser-produced spots and mass spectra have been obtained. The laser fluence ranges for elastic film deformation, film melting, and cracking have been determined. The laser-induced stress and temperature in titanium films have been evaluated. The Marangoni effect has been invoked to explain the formation of a well-defined ring of nanoparticles surrounding the molten zone in the film.
(6) The developed blister-based LIFT method has been applied to transfer fullerene C60 molecules from a metal-coated substrate into the extraction region of a time-of-flight mass spectrometer and a high efficiency of the transfer has been demonstrated. Based on measurements of the particle velocity distributions it has been shown that two mechanisms contribute to the LIFT process using ns laser pulses in the case of C60, mechanical “shake-off” due to laser-induced blister formation and thermal desorption.

The originally planned studies on the graphene-based sensor of neutral particles and on coupling of the laser ablation mass spectrometer with the graphene detector were unfortunately not performed for a number of technical and financial reasons. The research within the project was focused on detailed investigation and optimization of laser-based methods for gentle delivering of various particles into the detection region of analytical instruments. A crucial outcome of the project is the development of a robust and efficient matrix-free method for transfer of nano- and microparticles from metal surfaces into the gas phase. The developed blister-based LIFT technique is simple in realization and universal with respect to nanoparticle materials. It offers efficient isolation of transferred particles from any substrate ablation products and significant laser heating and thus delivers particles with their original composition. The method can be widely used in a variety of instruments for gas-phase analysis of different nanoparticles such as carbon nanostructures, aerosol particles, biological objects etc. Due to the relatively low velocities of the transferred particles (~50 m/s), the developed method allows the investigation of particle composition and morphology with depth resolution by applying a series of ultrashort laser pulses. This possibility was attempted with the Au-coated nanoparticles but due to the ablation properties of the gold coating a controlled nanoscale pulse-by-pulse removal was not possible. In order to explore this possibility in more detail some improvements require to be made to the apparatus and this could not be completed before the end of the project.

Although not all goals have been achieved at the present time, particularly in view of the risky program proposed by the project, the performed studies, the developed experimental techniques and methods, and established collaboration promise further progress in the field of nanoparticle detection and characterization. A general outcome of the Project is in making an important step from fundamental concepts of laser-based particle analysis in the gas phase to laboratory proof-of-principle studies and prototype development.