Light and Thermally Induced Charge Transfer and Ejection of Micro‐/Nanoparticles from Ferroelectric Crystal Surfaces

Abstract

pyroelectric (PY) effect, allowing handling and trapping particles being close to the ferroelectric platform. Numerous works have been reported on these techniques and their multiple applications. Some reviews or reference works are refs. [12,15,16] for photovoltaic optoelectronic tweezers (PVOT), and refs. [10,17,18] for PY trapping.The particle ejection phenomenon reported in this paper has been incidentally discovered during particle manipulation with PVOT. [19] The so-called PVOT are a rather recent technique that has undergone a rapid development in the last few years, [16] based on the electric fields generated by the bulk PV effect. This effect is a singular phenomenon which strongly appears in a few crystalline ferroelectric materials, [20,21] such as LiNbO 3 (LN), when properly doped (mainly Fe or Cu). It allows the generation of remarkably high electric fields (up to ≈1.5 × 10 5 V cm −1 ) [22] for moderate or low light excitation levels (around mW cm −2 ). The PV effect is associated with optical transitions from localized states of impurities, such as Fe 2+ /Fe 3+ or Cu + /Cu 2+ , and with a directional electron migration along the polar axis (PV current). Once displaced, the electrons are trapped in other defects and generate a spatial charge distribution and the corresponding electric field, [23] which are illustrated in Figure 1. Two crystal configurations (parallel and perpendicular), commonly used for particle manipulation, are shown. [24,25] They are defined by the orientation of the polar axis, parallel or perpendicular to the active surface, respectively. The PV electric field extends near the surface outside the crystal (evanescent PV fields), where it can act either on charged particles via electrophoretic forces, or on neutral objects polarized by the field via dielectrophoretic forces, [26,27] as illustrated in Figure 1.So far, the vast majority of the reported experimental results have been obtained by using a two-step sequential procedure: first, the PV substrate is illuminated, and then, the particles are manipulated and trapped by means of the light-induced PV electric fields. This sequential method can be employed because the PV fields remain after illumination for periods of time in the range of days to months unless they are thermally erased or externally screened. Although this method is quite convenient to trap particles and assemble permanent particle structures, other applications require real-time manipulation.During real-time operation of photovoltaic optoelectronic tweezers, a novel intriguing phenomenon has been recently observed, namely, silver nanoparticles previously deposited on ferroelectric LiNbO 3 :Fe surfaces are ejected from them by illumination. Here, it is shown that this phenomenon results from the electrical charging of the micro-/nanoparticles previously trapped on the LiNbO 3 :Fe surfaces and the subsequent Coulomb repulsion. Specific experiments are performed to determine the sign of the transferred charges, which is negative/positive for the +c/−c sample face...

All-Optical Domain Inversion in LiNbO3 Crystals by Visible Continuous-Wave Laser Irradiation

“…Indeed, the ability to generate customized electric field distributions by structured light, without external power supplies or lithography-patterned electrodes, makes Fe:LN a rather versatile and appealing asset. For example, Fe:LN has been successfully employed for the optofluidic manipulation of liquid droplets, − manipulation and patterning of micro/nanoparticles by PV optoelectronic tweezers, − guided locomotion and alignment of liquid crystals, − optical gating of graphene, or hybrid Fe:LN-graphene metasurfaces . All of these applications have been developed with monodomain Fe:LN crystals.…”

On-nanoparticle monolayers as a solute-specific, solvent-like phase

“…Self-assembled monolayers, SAMs, of organic ligands forming on the surfaces of nanoparticles have been studied widely as the means to modify, for instance, particles' solubility, [1][2][3][4] interparticle interactions, [5][6][7] electro-optical [8][9][10][11][12] and chargetransport [13][14][15][16][17][18][19] properties, and the ability to form covalent [20][21][22][23][24][25] or noncovalent [26][27][28][29][30][31][32] bonds with desired analytes or biomolecules. Recently, we 33,34 and others 25,[35][36][37][38][39][40][41][42][43][44] have used ligands terminated in charged end-groups to act as "gatekeepers," controlling the transport of small molecules from solution into the SAM's bulk.…”

Light‐Induced Virtual Electrodes for Microfluidic Droplet Electro‐Coalescence

“…Noncentrosymmetric ferroelectric crystals exhibit both properties, thus providing remarkable performance in micro-and nanomanipulation. [30][31][32][33] In particular, the PV effect enables the generation of space charge distributions inside the material shaped by structured light. [34] These space charge distributions act as virtual electrodes and allow several controlled operations on sessile droplets on the crystal [35][36][37] and also on confined single droplets within microfluidic channels.…”