LETTER pubs.acs.org/NanoLett
Thiocyanate-Capped PbS Nanocubes: Ambipolar Transport Enables Quantum Dot Based Circuits on a Flexible Substrate Weon-kyu Koh,† Sangameshwar R. Saudari,‡ Aaron T. Fafarman,‡ Cherie R. Kagan,*,†,‡,§ and Christopher B. Murray†,‡ †
Department of Chemistry, ‡Department of Materials Science & Engineering, and §Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
bS Supporting Information ABSTRACT: We report the use of thiocyanate as a ligand for lead sulfide (PbS) nanocubes for high-performance, thin-film electronics. PbS nanocubes, self-assembled into thin films and capped with the thiocyanate, exhibit ambipolar characteristics in field-effect transistors. The nearly balanced, high mobilities for electrons and holes enable the fabrication of CMOS-like inverters with promising gains of ∼22 from a single semiconductor material. The mild chemical treatment and low-temperature processing conditions are compatible with plastic substrates, allowing the realization of flexible, nonsintered quantum dot circuits. KEYWORDS: Nanocubes, quantum dots, field-effect transistors, inverters, circuits, flexible electronics, lead sulfide, thiocyanate ligands
C
olloidal inorganic nanocrystals (NCs) offer an attractive route to thin-film-based electronic devices.1 3 Organic surfactants present in typical colloidal syntheses promote solubility and provide precise size and shape control of NCs. Lead chalcogenides, an important class of low band gap semiconductors, are particularly amenable to these colloidal synthetic approaches, and spheres, rods, cubes and stars of exquisite monodispersity of size and shape are possible.4 7 However, these surfactants present large, bulky ligands on the NC surface, which when assembled into NC solids suppress interparticle coupling and make NC films electrically insulating, a limitation for device integration. Previous approaches for decreasing the interparticle distance in NC thin films to enhance charge transport have included thermal decomposition of the original ligand,8 10 or exchange for a smaller, more conductive ligand.1,2,11 18 Hydrazine and 1,2-ethanedithiol are common examples of small molecules that have been used to exchange the longer ligands used in NC synthesis and assembly to increase film conductivity and fabricate NC FETs1,11,12,17 and solar cells.2,13,14 However the extension of these results to the technologically important goal of creating high-performance, flexible electronics has been limited by the high reactivity of hydrazine,1,12,15,17 and the insufficient gains in charge carrier mobility for the other ligands.11,13,14,16 We recently introduced ammonium thiocyanate (SCN) as an alternative compact ligand for the postassembly treatment of NC films, yielding high-performance electronic devices.19 Herein we demonstrate that the noncaustic, environmentally benign nature of this treatment is compatible with common flexible substrates such as polyimide (DuPont Kapton). For these devices, discrete monolayers of close-packed NCs are fabricated to form the active layer, r 2011 American Chemical Society
using our recently introduced process for NC self-assembly at the liquid gas interface.20 The exquisite shape control attainable by colloidal synthesis allows us to take advantage of the high interparticle coupling energy and low void volume possible for closepacked cubic NCs. The NCs employed are quantum dots (QDs), meaning that the dimensions of the nanocubes (approximately 11 nm to a side) are well within the quantum confinement regime, given that the exciton Bohr radius of bulk PbS is 20 nm.21 After treatment with the SCN ligand and annealing at low temperatures (
