Band-like transport, high electron mobility and high photoconductivity in all-inorganic nanocrystal arrays Jong-Soo Lee 1 , Maksym V. Kovalenko 1 , Jing Huang 1 , Dae Sung Chung 1 and Dmitri V. Talapin 1,2 * Flexible, thin-film electronic and optoelectronic devices typi- cally involve a trade-off between performance and fabrication cost 1–3 . For example, solution-based deposition allows semicon- ductors to be patterned onto large-area substrates to make solar cells and displays, but the electron mobility in solution- deposited semiconductor layers is much lower than in semicon- ductors grown at high temperatures from the gas phase 4 . Here, we report band-like electron transport in arrays of colloidal cadmium selenide nanocrystals capped with the molecular metal chalcogenide complex 5,6 In 2 Se 4 22 , and measure electron mobilities as high as 16 cm 2 V 21 s 21 , which is about an order of magnitude higher than in the best solution-processed organic 7 and nanocrystal 8 devices so far. We also use CdSe/CdS core– shell nanoparticles with In 2 Se 4 22 ligands to build photodetec- tors with normalized detectivity D * > 1 3 10 13 Jones (I Jones 5 1 cm Hz 1/2 W 21 ), which is a record for II–VI nanocrys- tals. Our approach does not require high processing tempera- tures, and can be extended to different nanocrystals and inorganic surface ligands. Nanocrystal solids are considered to have potential as prospective materials for photovoltaics 9 , solid-state lighting 10 and thermoelec- trics 11 . All these applications rely on the efficient transport of charge carriers and require semiconducting materials with high carrier mobility m. Conduction in nanocrystal solids involves charge transfer between individual nanocrystals and is strongly dependent on the rate of electron tunnelling through the interparticle medium 8,12,13 . Surface ligands with long hydrocarbon chains, typi- cally used for nanocrystal synthesis, form highly insulating barriers around each nanocrystal. The electronic coupling between adjacent nanocrystals can be increased by replacing bulky ligands with smaller capping molecules such as pyridine, n-butylamine and so on 9,14 . Furthermore, chemical treatments of nanocrystal solids with dilute solutions of hydrazine, methylamine and 1,2-ethanedithiol can significantly improve m (up to ≏1 cm 2 V 21 s 21 ; refs 12,15–20). Our group has recently reported that various molecular metal chal- cogenide complexes (MCCs), such as Sn 2 S 6 42 , could serve as capping ligands for a broad range of metallic and semiconducting nanocrystals and nanowires 5 . As a result of their small size and appropriate highest occupied molecular orbital (HOMO) and lowest unoccupied mole- cular orbital (LUMO) energies, MCC ligands facilitate strong electronic coupling in nanocrystal solids; this has been demonstrated by the 200 S cm 21 conductivity measured in films of 5 nm gold nanocrystals capped with Sn 2 S 6 42 ligands 5 . At the same time, Sn 2 S 6 42 -capped CdSe nanocrystals show only a modest electron mobility (m e ) of 3 × 10 22 cm 2 V 21 s 21 (ref. 5). In this work we show that appropriate combinations of nanocrystals, MCC ligands and device structure can improve m e by several orders of magnitude, leading to band-like charge transport in nanocrystal solids. In this work, we used MCC ligands prepared by co-dissolution of In 2 Se 3 and selenium in anhydrous hydrazine at room temperature. Mitzi and colleagues have proposed that the MCCs obtained in this way have a (N 2 H 4 ) 2 (N 2 H 5 ) 2 In 2 Se 4 composition, with In 2 Se 4 22 nominal structural units 2 . These MCCs displaced the organic mol- ecules at the surface of CdSe and CdSe/CdS core–shell nanocrystals originally capped with n-octadecylphosphonate and oleate ligands, respectively (see Methods). No changes in nanocrystal size and 400 500 600 700 CdSe/CdS NCs Absorbance (a.u.) Wavelength (nm) Organic ligands In 2 Se 4 2- ligands CdSe NCs c Wavenumbers (cm -1 ) 3,500 3,000 2,500 2,000 1,500 1,000 CdSe NCs-In 2 Se 3 Transmission (a.u.) CdSe NCs-organic ligands CdSe NCs-(N 2 H 4 ) 2 (N 2 H 5 ) 2 In 2 Se 4 d a 50 nm b 50 nm Figure 1 | Colloidal CdSe and CdSe/CdS nanocrystals with indium selenide capping ligands. a,b, Transmission electron microscopy (TEM) images of CdSe (a) and CdSe/CdS (b) nanocrystals capped with (N 2 H 4 ) 2 (N 2 H 5 ) 2 In 2 Se 4 . c, Absorption spectra for 4.6 nm CdSe nanocrystals and 10.0 nm CdSe/CdS core–shell nanocrystals capped with original organic capping ligands in hexane (black lines) and capped with In 2 Se 4 22 in hydrazine (blue lines). Inset: photographs of colloidal solutions of CdSe (left) and CdSe/CdS (right) nanocrystals capped with In 2 Se 4 22 . d, Fourier transform infrared spectra for 4.6 nm CdSe nanocrystals capped with the original organic ligands (black line), and with (N 2 H 4 ) 2 (N 2 H 5 ) 2 In 2 Se 4 ligands before (red line) and after (blue line) annealing at 200 8C. The IR spectra were normalized to the amount of absorbing material and vertically shifted for clarity. NC, nanocrystal. 1 Department of Chemistry and James Frank Institute, University of Chicago, Chicago, Illinois 60637, USA, 2 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA. *e-mail: dvtalapin@uchicago.edu LETTERS PUBLISHED ONLINE: 24 APRIL 2011 | DOI: 10.1038/NNANO.2011.46 NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology 1 © 2011 Macmillan Publishers Limited. All rights reserved.