Cross-platform characterization of electron tunneling in molecular self-assembled monolayers☆
Introduction
Characterization of charge transport in molecular scale electronic devices has to date shown exquisite sensitivity to specifics of device fabrication and preparation. Thus, intrinsic molecular band structure has been problematic to extract from published results. Here we investigate cross-platform device characterization through a large bandgap alkanethiol self-assembled monolayer (SAM) devices fabricated in different methods and platforms.
When a molecular layer with a large HOMO–LUMO gap (HOMO: highest occupied molecular orbital, LUMO: lowest unoccupied molecular orbital) is sandwiched between two metal contacts, a well-defined metal–insulator–metal (M–I–M) tunneling is expected. One molecular system that has been studied extensively is the alkanethiol [CH3(CH2)n−1SH] SAM [1]. Scanning tunneling microscopy [2], conducting atomic force microscopy (CAFM) [3], [4], mercury-drop junctions [5], [6], and cross-wire junctions [7] have been used to investigate electron transport through alkanethiol SAMs. The electron conduction through alkanethiol SAMs is expected to be tunneling because the Fermi levels of the contacts lie within the large HOMO–LUMO gap (∼8 eV) of short molecular length (1–2.5 nm) for the case of these alkanethiols [8]. Although it has been recently shown that tunneling is the main conduction mechanism through the alkanethiol SAMs from temperature-independent current–voltage [I(V)] measurement results [8], the tunneling current densities and the electrical parameters determining the tunneling characteristics such as the tunneling barrier height and the decay coefficient vary among the different measurement techniques [2], [3], [4], [5], [6], [8] which can be due to different geometry, junction area uncertainty, and device-to-device variation.
In this study, tunneling characteristics through alkanethiol SAMs are investigated using three types of device structures: nanometer scale and micrometer scale device structures and a structure for CAFM study. The measured I(V) data are compared with theoretical calculations of the M–I–M tunneling model. I(V) measurements on various alkanethiols of different molecular lengths are also performed for the study of length-dependent tunneling behavior.
Section snippets
Alkanethiol deposition
For our experiments, a∼5 mM alkanethiol solution was prepared by adding ∼10 μl alkanethiols into 10 ml ethanol. The deposition was done in solution for 1–2 days inside nitrogen filled glove box with an oxygen level of less than 100 ppm (or 1 ppm). Alkanethiols of different molecular lengths: octanethiol [CH3(CH2)7SH; denoted as C8, for the number of alkyl units], decanethiol [CH3(CH2)9SH; C10], dodecanethiol [CH3(CH2)11SH; C12], tetradecanethiol [CH3(CH2)13SH; C14], hexadecanethiol [CH3(CH2)15
Method 1. Nanoscale devices
Fig. 2 shows representative I(V) characteristics of C8, C12, and C16 SAMs measured (symbols) with the device structure as shown in Fig. 1(a). By using the contact area of 45 ± 2 nm (for C12 and C16 devices) [8] and 50 ± 8 nm (for C8 device) in diameter, current densities of 31,000 ± 10,000, 1500 ± 200, and 23 ± 2 A/cm2 at 1.0 V are determined for C8, C12, and C16, respectively. From the temperature-independent current–voltage measurement results, tunneling was shown to be the main conduction mechanism
Conclusions
Electron tunneling through alkanethiol SAMs was investigated using three different structures. The tunneling conduction was examined by varying molecular length. Although all the three structures showed exponential dependence on the molecular length with a decay coefficient, alkanethiol SAMs formed in the nanometer scale structure were the only ones in which tunneling transport is unambiguously shown; i.e., temperature-independent and length-dependent conduction. Although other methods can give
Acknowledgements
The authors would like to thank Ilona Kretzschmar, Azucena A. Munden, and Ryan Munden for helpful discussions. This work was supported by DARPA/ONR (N00014-01-1-0657), ARO (DAAD19-01-1-0592), AFOSR (F49620-01-1-0358), NSF (DMR-0095215), and NASA (NCC2-1363).
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Original version presented at the 4th International Conference on Electroluminescence of Molecular Materials and Related Phenomena (ICEL4), 27–30 August 2003, Cheju Island, Korea.