Nanometer-scale crystals of group IV, III-V, II-VI, IV-VI, I-III-VI₂, and metal-halide perovskite semiconductors, capped by organic or inorganic ligands and dispersed in solvents, are known as colloidal nanocrystals (NCs) [Figure 1A,B] [1], [2]. Colloidal NCs form an excellent, solution-processable materials class for thin film and flexible electronics [Figure 1C-E]. In this tutorial, I will begin by describing the device physics of the thin-film, field-effect transistor. The field-effect transistor is a three-terminal, semiconductor device having metallic source, drain, and gate electrodes, a semiconductor channel (e.g., in this case a NC thin film), and gate dielectric layer which separates the gate from the semiconductor channel [Figure 1D]. Applying a voltage to the gate electrode creates an electric field across the gate dielectric material and modulates the carrier concentration and thus current between source and drain electrodes. I will then review the size, composition, and surface chemistry-dependent physical properties of semiconductor NCs and the arrangement and interparticle distance of NCs in thin films [Figure 1C], and connect these physio-chemical characteristics to the electronic properties of NC thin films and ultimately to advances reported in the performance of NC field-effect transistors. Next, I will share an accessible introduction to NC field-effect transistors as building blocks of electronic circuitry, giving examples of analog and digital circuit topologies and their functions. Device design and fabrication methods will be described that have enabled the scaling up in complexity and area and scaling down in device size of flexible, colloidal NC integrated circuits. Finally, taking stock of the advances made in the science and engineering of NC systems, I will describe challenges and opportunities to develop next-generation, colloidal NC electronic materials and devices, important to their potential in future computational and in Internet of Things applications.

Figure 1. (A) Schematic of a colloidal semiconductor NC composed of a nanometer-scale crystalline semiconductor core capped by ligands. (B) Photograph of a colloidal NC dispersion and (C) SEM images of glassy and ordered NC thin films. (D) Schematic of NC field-effect transistor device structure and (E) photograph of analog and digital NC integrated circuits fabricated on a 4” diameter flexible substrate.