DOI: 10.1002/adma.200700309 A Bit per Particle: Electrostatic Assembly of CdSe Quantum Dots as Memory Elements** By Bikas C. Das , Sudip K. Batabyal, and Amlan J. Pal* Semiconducting nanostructures exhibit attractive electrical, optical, and magnetic properties that are different from those of their bulk phase. Quantum dots, being quasi-zero-dimen- sional, have a sharper density of states than higher-dimen- sional structures. They hence have superior transport and op- tical properties and are thus suitable for various electronic and optoelectronic applications. [1–3] Because such properties are size-dependent, it is customary to vary the size of the par- ticles to monitor device characteristics for novel applications. In recent years, gold and semiconducting nanoparticles, em- bedded in polymer matrices, exhibited electrical bistability for memory applications. [4–7] Electric-field-induced charge trans- fer followed by charge confinement in nanoparticles led to a high-conducting state. Device dimensions (thickness ranging from 50 nm to a couple of hundred of nanometers), however, restricted the use of these nanoparticles in high-density mem- ory elements. A two-dimensional array of nanoparticles can be achieved with the advent of layer-by-layer (LbL) electrostatic assem- bly. [7–9] In LbL films, the nanoparticles are electrostatically ad- sorbed because of suitable functional groups present in the stabilizer. They hence form a two-dimensional assembly. In such two-dimensional arrays, the properties of a single parti- cle or dot (or an assembly of them) can be studied. In this arti- cle, we present our in-depth work on the electrical bistability in two-dimensional arrays of CdSe nanoparticles (with parti- cles sizes ranging down to the quantum dot regime) in achiev- ing a bit per particle. The composition of the nanoparticles was evaluated by using X-ray diffraction (XRD), and high-resolution transmis- sion electron microscopy (HRTEM) images; their sizes were measured from TEM images and electronic absorption spec- tra. XRD spectra showed diffraction patterns corresponding to CdSe crystals (Supporting Information, Fig. S1). HRTEM images of the nanoparticles showed lattice spacing corre- sponding to CdSe crystals, confirming the formation of CdSe nanoparticles (Supporting Information, Fig. S2). Infrared ab- sorption spectra of the mercaptoacetic acid-capped particles showed the presence of vibrations corresponding to the li- gands attached to the nanoparticles (Supporting Information, Fig. S3). TEM images of the nanoparticles, adsorbed on car- bon-coated grids, showed nearly spherical particles (Support- ing Information, Fig. S2). Scanning electron microscopy (SEM) images of the nanoparticles electrostatically adsorbed on a Si substrate show that the substrate was covered uni- formly with the nanoparticles (Supporting Information, Fig. S4). UV-vis electronic absorption spectra of four different CdSe nanoparticles dispersed in water are shown in Figure 1a. Each of the spectra shows a single peak. The peak wavelengths for the four nanoparticles is 372, 418, 474, and 508 nm, as is also evidenced by a photograph of the solutions (inset of Fig. 1a). The wavelengths correspond to a particle size of 3.5, 4.6, 6.7, and 8.7 nm for CdSe. In other words, larger CdSe particles have a smaller band gap. The sizes were obtained from the equation [10] R  h E gn  E gb   E gb 2m    s 1 where R is the particle size, h is the Plank constant, m * is the effective mass of the electron ( = 0.1 × the rest mass of the electron), and E gb and E gn are the band gaps for the bulk (= 1.84 eV) and nanoparticles, respectively. The size of the CdSe nanoparticles hence ranges down to the quantum dot re- gime. We recorded the electronic absorption spectrum of a mono- layer of CdSe nanoparticles on a quartz substrate. Because the value of absorbance was low for a monolayer, we con- firmed deposition of the particles by monitoring the progress of the electrostatic adsorption process (LbL deposition) dur- ing multilayer deposition. For the 4.6 nm particles, electronic absorption spectra of a different number of layers of LbL films are shown in Figure 1b. All the five spectra showed a peak at 439 nm—the intensity of the band increasing with the number of layers deposited. The absorption band is red- shifted compared to that measured for 4.6 nm particles in dis- persed solution. Because the 439 nm band corresponds to a particle size of 5.3 nm, which is not an integer multiple of 4.6 nm, the red-shift in the absorption spectrum cannot be caused by the formation of clusters in the films. The shift COMMUNICATION 4172 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2007, 19, 4172–4176 – [*] Prof. A. J. Pal, B. C. Das, S. K. Batabyal Indian Association for the Cultivation of Science Department of Solid State Physics Kolkata 700032 (India) E-mail: sspajp@iacs.res.in [**] BCD acknowledges a CSIR Junior Research Fellowship No. 9/ 80(504)/2006-EMR-I, Roll No. 503982. The Department of Science and Technology of the Government of India financially supported the work through a Ramanna Fellowship SR/S2/RFCMP-02/2005. Supporting Information is available online from Wiley InterScience or from the author.