Optical and optoelectronic techniques for micro‐ and nano‐object manipulation are becoming essential tools in nano‐ and biotechnology. Among optoelectronic manipulation platforms, photovoltaic optoelectronic tweezers (PVOTs) are an emergent technique that are particularly successful at producing permanent nanoparticle microstructures. New strategies to enhance the capabilities of PVOT, based on real‐time operation, are investigated. This optoelectronic platform uses z‐cut LiNbO3:Fe substrates under excitation by a Gaussian light beam. Unexpected results show that during illumination, metallic particles previously deposited on the substrate are ejected from the light spot region. This behavior differs from the trapping phenomenon observed in previous work on PVOT operation, using a sequential method in which illumination is prior to particle manipulation. To discuss the results, a novel mechanism of charge exchange between particles and the ferroelectric substrate is proposed. Applications of this repulsion behavior are investigated. On the one hand, either particle repulsion or trapping in the illuminated region can be obtained by simply light switching on/off. On the other hand, by moving the light spot, different kinds of arbitrarily shaped tracks along the light path, either empty or filled with particles, are obtained. The results demonstrate new key capabilities of PVOT, such as pattern drawing, erasure, and reconfiguration.
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...
Photovoltaic optoelectronic tweezers (PVOTs) have been proven to be an efficient tool for the manipulation and massive assembly of micro/nano-objects. The technique relies on strong electric fields produced by certain ferroelectric materials upon illumination due to the bulk photovoltaic effect (customarily LiNbO3:Fe). Despite the rapid development of PVOTs and the achievement of high-quality 1D and 2D particle patterning, research efforts aimed at the fabrication of combinatorial structures made up of multiple types of particles have been scarce. Here, we have established the working principles of three different methods to tackle this pending challenge. To that end, dielectrophoresis and/or electrophoresis acting on neutral and charged particles, respectively, have been suitably exploited. Simple mixed structures combining metallic and dielectric nanoparticles of different sizes have been obtained. The results lay the groundwork for future fabrication of more complex combinatorial structures by PVOT, where micro/nanoparticles are the basic building blocks of miniaturized functional devices.
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