Laser-induced wavelength-controlled self-assembly of colloidal quasi-resonant quantum dots

Abstract

1. Introduction Irradiation of ensembles of micro-and nanoparticles with optical radiation leads to various effects allowing the control of their motion. One of these effects is optical binding, being the effect based on optically induced interaction between particles [1], in contrast to optical trapping [2,3] which is based on forces raised from interaction of individual particles with light. In optical binding effect, electric field of a light wave polarizes particles, and ac polarizations of particles interact each other. For interparticle distances much smaller than the wavelength this can be treated as the local field effect, i.e. external ac electric field acting onto individual particle becomes modified by the account of the field produced by neighboring particles. Optically induced interaction of particles becomes noticeable at certain level of light intensity when the potential energy associated with this interaction is comparable with kinetic energy of Brownian motion or with the potential energy of a barrier typically used to stabilize colloid solutions of NPs' ensembles. In recent studies, the most attention was devoted to the investigation of metallic NPs interacting with light. Particularly, calculation technique for modeling the dynamics of metal NPs was elaborated [4]. High precision manipulation of multiple metallic NPs using optical binding force induced by off-resonant low-power laser was recently demonstrated at distances up to several microns [5]. Alternatively, self-assembly of metal NPS at the subwavelength scale due to plasmonic interaction was obtained under strong off-resonant cw laser illumination, as was deduced from Raman and Rayleigh scattering [6]. Somehow similar result was observed in [7] where hybridization of metal NPs plasmonic resonance was observed in scattering when NPs were confined in an optical trap. Employing the plasmonic resonance of metal NPs with the laser frequency is widely recognized as means for enhancement of optically induced interaction between metallic NPs, as theoretically investigated [8,9]. Moreover, it was shown in these two papers that formation of NPs' clusters with pre-defined geometry is possible via proper choice of laser wavelength and polarization. Characteristic potential well depths for silver NPs, according to [9], are less than ten energies of thermal motion at laser field strengths close to metal NPs' breakdown threshold. More deep potential wells could be obtained when NPs with narrower resonant widths are under the self-assembly. Resonant widths of excitons in semiconductor QDs are two orders of magnitude narrower than plasmonic resonances in metals [10]. Formation of QDs' structures under optical irradiation due to various non-electrodynamical mechanisms were studied, e.g. in [11][12][13]. Surprisingly, no experimental attempts were done up to date to perform electrodynamically-driven laser-induced self-assembly of semiconductor nanoparticles, that may be of interest for quantum information, spintronics, single-electron transport, and sens...

Three-dimensional model of quantum dots' self-assembly under the action of laser radiation

“…Radius of particles including the absorbed layer was r = 6 nm, the density r = 5.81 g/cm 3 , electric dipole moment of the first excitonic transition |d12| 2 = 2•10 -46 J•m 3 . These particles' parameters corresponded to those of CdTe quantum dots used in the experimental study of the authors [10]. Water viscosity was taken to be η = 8.9•10 -5 P, dielectric constant ε = 81; laser field strength in all calculations was taken to be E0 = 6•10 6 V/m.…”

“…Pulsed lasers with microsecond pulse duration with the possibility of tuning in the broad wavelength range necessary for efficient assembly do not exist. Due to this fact, the experiments in [10] were conducted using tunable optical parametric oscillator with the required power and pulse duration 10 nm. Therefore, it is worthy to estimate the probability of pair formation during a single 10 nm pulse (Fig.…”

“…Therefore, using repeated pulsed action allows achieving formation of acceptable number of structures, which was observed in the experiment. [10] (after 3000 pulses approximately 40 % of particles were assembled into pairs).…”

“…Topology and content of forming nanostructure depend on the wavelength and the polarization of external field. The first experiments [10] have shown the principal possibility of pairs' formation from an ensemble of chaotic colloidal quantum dots. It was demonstrated that if the wavelength is chosen to correspond the maximum of dipole-dipole attraction of a pair of particles then up to 50 % of initially solitary particles become aggregated into pairs.…”

Comparative Analysis of Methods for Enhancement of the Photostability of CdTe@TGA QD Colloid Solutions

“…Such the regimes occur, for example, when QDs are utilized for cellular labeling and biological imaging, 14,15 or in the processes of self-assembly of QDs into the structures of pre-defined geometry in the field of laser radiation. 16 The role of dissolved oxygen in the photoinduced instability of colloidal QDs is well known. 14 Ma Serum Albumin (BSA) is well studied and widely used protein which enables modification of nanoparticles surfaces with the size of several tens of nm 17,18 or for their binding.…”