Tunnel diodes fabricated from CdSe nanocrystal monolayers

Abstract

A parallel approach for fabricating nanocrystal-based semiconductor-insulator-metal tunnel diodes is presented. The devices consisted of a Au electrode, a monolayer of 38 Å CdSe nanocrystals, an insulating bilayer of eicosanoic acid (C 19 H 39 CO 2 H), and an Al electrode. Each device was approximately 100 m 2 . Conductance measurements at 77 K reveal strong diode behavior and evidence of Coulomb blockade and staircase structure. A single barrier model was found to reproduce the electronic characteristics of these devices. © 1999 American Institute of Physics. ͓S0003-6951͑99͒04502-7͔The unique chemical and size-dependent properties of nanocrystals ͑NCs͒ have made them attractive candidates as electronic and photonic materials. Relatively straightforward fabrication procedures have been reported for certain NCbased devices, such as light-emitting diodes, 1 photovoltaics, 2 and capacitors. 3 Other devices, however, such as singleelectron transistors 4 and single-electron tunnel junctions, 5 require two electrical contacts to a single particle, and are hard to fabricate. 6 The difficulty arises not just because of the length scales involved, but also because chemically fabricated NCs are unstable toward many electronic materials processing technologies. In this letter, we present a parallel fabrication technique for the construction of CdSe nanocrystal-based metal-insulator-semiconductor ͑MIS͒ tunnel diodes. The key component that allows for top and bottom contacts to the nanocrystal ͑mono͒ layer is a molecularly thin insulating layer of eicosanoic acid, prepared as a Langmuir-Blodgett ͑LB͒ film. Strong diode behavior and a series of single-electron charging states, characterize the conductance of the devices. A standard model for MIS devices, 7 modified to include the finite-size characteristics of the NCs, successfully reproduced the measured conductance of the diodes. Figure 1͑a͒ shows a cross sectional view of the device. It consists of a Au electrode, NC monolayer, molecular insulator, and a top Al electrode. The structure resembles a normal metal-insulator-semiconductor diode, in which a CdSe NC monolayer 8 replaces the semiconductor, and a molecular ͑LB͒ film replaces what is, typically, an oxide tunnel barrier.Other groups have investigated the insulating behavior of LB films of organic amphiphiles in sandwich-structure devices. 9,10 Because of defects in the LB film, it is necessary to transfer several monolayers to fabricate what are, effectively, single-monolayer devices. We found that, for ϳ100 m 2 devices, 6 LB monolayers of eicosanoic acid gave an ''effective'' insulating layer of 2-3 nm ͑about 1 ML͒, and a device resistance of ϳ10 M⍀.38Ϯ4 Å CdSe NCs were synthesized according to literature recipes, 11 ligand-exchanged with hexanethiol, and characterized by UV/vis absorption spectroscopy and x-ray powder diffraction. A particle/HCCl 3 solution was spread on the a͒ Corresponding author; electronic mail: heath@chem.ucla.edu FIG. 1. Cross-sectional diagram ͑a͒ and energy-level diagram ͑b͒ of the CdSe na...

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