Fluorescence Microscopy Characterization of Patterned Nano-Crystalline Quantum Dot Films

Abstract

This experiment is a continuation of the study concerned with the lift-off based patterning of nanocrystaline quantum dot (NQD) films [1]. The ultra-thin films formed using colloidal solutions of nanocrystalline quantum dots show good promise in display/LED, as well as nano-labeling applications between others. Regardless of application, to become a part of the functional device the NQD film must be patterned accordingly. Following on the experiments demonstrating effectiveness of the lift-off process in patterning NQD films with resolution down to 1 µm [1] further investigation of the performance of the proposed patterning scheme needed to be carried out. The goal of this contribution is to study uniformity of surface coverage with lift-off patterned NQD films by means of fluorescence microscopy. When exposed to UV light the NQDs feature high photoluminescence with intensity depending on the dots size. The fluorescence microscopy allows capturing the intensity to obtain information about the NQD film uniformity. As a result, using images obtained by means of fluorescence microscopy and employing dedicated image processing tools information regarding film uniformity, pattern definition accuracy, and average intensity can be derived. Also, the fluorescence image can be transferred into grey scale to study its histogram plot and get information about intensity distribution across the NQD patterns. In this study silicon wafers used as substrates are first covered by spin coating with a layer of positive 1805 photoresist to the thickness of about 420 nm and then cured at 100oC. The resist covered wafers were subsequently exposed through the standard Cr-photomask in an aligner/exposure and developed in photoresist developer (351) to create the required patterns [1]. After that, NQDs were blanked deposited using mist deposition technique. The NQD deposited were CdSe/ZnS core/shell featuring average diameter of 6.5 nm and corresponding red (620 nm) emission wavelength. Then, the photoresist removal performed using either dry or wet lift-off. The dry lift-off applied using O2 plasma system and the wet lift-off used acetone dissolution [2]. Figure 1 demonstrates the fluorescence image for NQD patterned film and its intensity distribution. The intensity analysis of the patterns after the lift-off process is considered in terms of an average intensity. The results obtained in this study give further insights into the process of lift-off patterning of nano-crytalline quantum dot films and demonstrate feasibility of the proposed approach. In the full account of this work a complete set of experimental results obtained in the course of this investigation is presented and more detailed analysis of fluorescence images provided.