J. Phys. Chem. B 2006, 110, 18231-18235
18231
Electrical Bistability in Electrostatic Assemblies of CdSe Nanoparticles Kallol Mohanta, Swarup K. Majee, Sudip K. Batabyal, and Amlan J. Pal* Department of Solid State Physics, Indian Association for the CultiVation of Science, Kolkata 700 032, India ReceiVed: June 26, 2006; In Final Form: July 27, 2006
We report electrical bistability in electrostatic assembly of CdSe nanoparticles. We obtained thin films of the nanoparticles via layer-by-layer electrostatic assembly technique, which provided a nanoscale control to tune the thickness. Devices based on such thin films exhibit electrical bistability along with memory phenomenon. The bistability is due to charge confinement in the nanoparticles. Conduction mechanism changes from an injection-dominated to a bulk one during switching from a low- to a high-conducting state. Additionally, results from impedance spectroscopy show that the dielectric constant of the material increases during the transition. Both random-access and read-only memory applications are observed in these systems.
Introduction In recent years growth and characterization of semiconducting nanoparticles have been widely studied.1-3 Their electrical, optical, and magnetic properties are somewhat different as compared to those of the bulk systems. The special physical and chemical properties of the nanoparticles are considered to be due to the quantum confinement effect.4-6 Hence, the sizes of such nanomaterials are varied systematically to study quantum size effects or to make novel electronic or optical materials with tailored properties.7,8 In this direction, a couple of research groups pioneered the studies of electrical9,10 and optical11,12 properties of individual nanocrystals. Apart from unique physical properties, nanoparticles now exhibit interesting applications also. With their advantages in size and dimension, they are ideal for data storage or memory applications to provide high-density memory elements.13 Memory phenomenon in nanoparticles arises due to their electrical bistability, which is triggered by charge confinement via a suitable voltage pulse. In such cases, current-voltage (I-V) characteristics depend on the direction of voltage sweep. A higher level of conductivity can also be achieved by applying a suitable voltage pulse. Electrical bistability in gold nanoparticle/organic material composites in a polymer matrix was observed in 2004.14 A memory device with triple-layer structure has been reported where a discontinuous metal layer was incorporated between two organic layers.15 A polymer nanofiber decorated with gold nanoparticle has also been realized as active memory elements.16 Electric field induced charge transfer between nanoparticle and nanofiber has been attributed to be operative in electrical bistable and memory devices.17 Nanoscale control of material properties is a challenging job for modern technologies. One such method for nanoscale construction, the layer-by-layer technique (LbL), has already proven effective for the preparation of functional thin films.18 In Langmuir-Blodgett or LbL films of nanoparticles,19 they can be confined in a controlled way. With respect to memory applications, electrostatically bound nanoparticles (via the LbL technique) will mean that the density of memory bits can be controlled by the density of confined (and isolated) nanopar* To whom correspondence
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ticles. With a futuristic aim to achieve one bit/particle, we put CdSe nanoparticles in restricted geometries via the LbL technique. The nanoparticles, which are confined in association with an inert polycation, are characterized for electrical bistability and memory applications. Experimental Section Cadmium acetate (Cd(COOH)2‚2H2O), selenium (Se), sodium sulfite (Na2SO3), sodium hydroxide (NaOH), mercaptoacetic acid (HSCH2COOH), and poly(allylamine hydrochloride) (PAH, MW ) 70 000) were purchased from Aldrich and used without further purification. While cadmium acetate and sodium selenosulfate provided the required sources for cadmium and selenium ions, respectively, mercaptoacetic acid was used as a surface capping agent. In a typical synthesis we prepared aqueous solutions of cadmium acetate (66.6 mg in 25 mL) and mercaptoacetic acid (15 mg in 20 mL). The solutions were thoroughly mixed, and the pH of mixed solutions was adjusted to 10.5 by adding dilute NaOH. Sodium selenosulfate (Na2SeSO3) solution was prepared by refluxing 2 g of selenium and 4.83 g of sodium sulfite in 100 mL of water for 10 h.20 After the residual selenium was filtered off, 2 mL of the freshly prepared solution was added to the cadmium ion source-mercaptoacetic acid mixed solution. Within 30 min, the transparent solution became greenish yellow and resulted in acid-stabilized CdSe nanoparticles dispersed in water. The initial molar ratio of Cd2+-Se2--mercaptoacetic acid was 4:5:2. The yellowish solution was cooled to 5 °C and centrifuged at 14 000 rpm. The resulting powder was then dried in a vacuum oven at 60 °C. The acid-stabilized nanoparticles were finally dispersed in deionized water to get an optically homogeneous solution, which was used as an anionic bath for LbL deposition. A 5 mM PAH solution (pH ) 6.5) was the cationic bath for LbL film deposition. To deposit LbL films of the CdSe nanoparticles, quartz and patterned indium tin oxide (ITO) coated glass substrates were first dipped in the PAH solution for 15 min. The slides were then washed in three deionized water baths for 2, 2, and 1 min, respectively. They were then immersed in the CdSe bath for 15 min. The -COO- groups of the nanoparticles were bound to the -NH3+ sites of PAH to lead to adsorption of nanoparticles. The slides were then washed in water by following the
10.1021/jp0639795 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/02/2006
18232 J. Phys. Chem. B, Vol. 110, No. 37, 2006
Mohanta et al. interface bus (GPIB), and measurement was carried out with LabView/SMaRT softwares. Results and Discussion LbL Films of the Nanoparticles. The absorbance spectrum of CdSe nanoparticles dispersed in water is shown in Figure 1. From the band position, the average diameter of the particles has been estimated from the equation21
R)
Figure 1. Electronic absorption spectra of CdSe in dispersed solution and LbL films of different bilayers (as shown in the legends). The inset shows the absorbance of the films at 440 nm as a function of number of bilayers.
same rinsing protocol. This resulted in one bilayer of LbL film of CdSe nanoparticles. Repetition of the dipping procedure in sequence resulted in the desired number of bilayers on the slides. The films were annealed in a vacuum at 120 °C for 2 h. To study the morphology of the films, field emission scanning electron microscope (FESEM) images were recorded through a JEOL JSM-6700F instrument. Films on quartz substrates were used to record UV-vis absorption spectra. The films on ITO electrodes were, on the other hand, used for electrical characterization. Aluminum (Al) as a top electrode was thermally deposited in a vacuum (
