Direct patterning of quantum dot nanostructures via electron beam lithography† Vikas Nandwana, a Chandramouleeswaran Subramani, a Yi-Cheun Yeh, a Boqian Yang, a Stefan Dickert, b Michael D. Barnes, ab Mark T. Tuominen b and Vincent M. Rotello * a Received 22nd April 2011, Accepted 8th July 2011 DOI: 10.1039/c1jm11782c Patterned quantum dot (QD) nanostructures were prepared by direct electron beam lithography on QD films. Time resolved pho- toluminescence measurements show that the optical properties of these QDs were retained after cross-linking. Quantum dots (QDs) provide tunable optical and electrical proper- ties, 1 making them useful for optoelectronic devices such as light- emitting diodes, 2 photodetectors, 3 field-effect transistors, 4 photovol- taic cells, 5 and photorefractives. 6 Patterning of QDs is critical for their integration with solid-state devices. Patterned surfaces of QDs have been used in photonics, 7 light-emitting devices, 8 and hybrid organic/ inorganic solar cells. 9 Recent advances include the generation of two- and three-dimensional patterns through recognition directed self- assembly, 10 micro contact printing, 11 and other lithographic techniques. 12 Unlike conventional lithography, direct patterning offers a substantial simplification in processing by using the functional component as a negative resist. Direct patterning avoids the complications of resist deposition, etching and removal, thus avoid- ing issues of resist/film adhesion, long processing times and roughness generation during the etching step. Electron beam lithography (EBL) is an attractive lithographic technique for direct patterning, allowing fabrication of arbitrary nanostructures <50 nm with high accuracy. 13 Direct patterning via EBL offers a simple yet highly accurate approach that has been used to pattern Au nanostrctures. 14 Herein, we report a facile strategy of direct patterning QD nano- structures via EBL (Scheme 1). First, films of trioctylphosphine oxide (TOPO) functionalized CdSe/ZnS QDs were deposited onto a gold coated silicon substrate by spin-coating, allowing precise control over the film thickness and enabling us to cover large areas with unifor- mity. Next, the pattern was written directly in the QD film using EBL. Finally, unexposed QDs were removed by washing with toluene, leaving the exposed areas of the QD film firmly anchored to the substrate due to e-beam induced crosslinking of organic ligands. These materials retain the optical properties of the unpatterned films: QD fluorescence intensities and lifetimes were essentially unchanged before and after exposure. As a preliminary step for EBL processing, we optimized the dosage to provide efficient crosslinking of QDs. For this purpose, we explored a range of e-beam dosage exposures to obtain the right dosage required to generate stable QDs features without excessive degradation and overexposure of QD. The test pattern was written in 55 nm thick QD films in the form of boxes of size 5  5 mm 2 while the dosage was varied from 100 mC cm 2 to 8000 mC cm 2 . The film thickness of 55 nm was kept constant throughout all the experiments, as thicker films had relatively high surface roughness. The fluorescent image and intensity map of QD test pattern before developing (Fig. 1(a) and (b)) show a successive drop in fluorescent intesities with increasing e-beam dosages. Fig. 1(c) and (d) show bright field and fluorescent image of the test pattern after washing with toluene. It was found that at very low dosages, the fluorescence Scheme 1 Direct patterning of QD nanostructures by electron beam lithography on QD films. a Department of Chemistry, University of Massachusetts Amherst, 710 North Pleasant Street, Amherst, MA, 01003, USA. E-mail: rotello@ chem.umass.edu; Fax: +1 413-5452058 b Department of Physics, University of Massachusetts Amherst, Amherst, MA, 01003, USA † Electronic supplementary information (ESI) available: QD synthesis, absorption and emission spectra, XPS data, fluorescence intensity map of developed QD test pattern after washing and emission spectra of QD film before and after e-beam exposure. See DOI: 10.1039/c1jm11782c This journal is ª The Royal Society of Chemistry 2011 J. Mater. Chem., 2011, 21, 16859–16862 | 16859 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. Chem., 2011, 21, 16859 www.rsc.org/materials COMMUNICATION Downloaded by University of Massachusetts - Amherst on 15 August 2012 Published on 25 July 2011 on http://pubs.rsc.org | doi:10.1039/C1JM11782C View Online / Journal Homepage / Table of Contents for this issue