Structure Matters: Correlating temperature dependent electrical transport through alkyl monolayers with vibrational and photoelectron spectroscopies

Hagay Shpaisman; Oliver Seitz; Omer Yaffe; Katy Roodenko; Luc Scheres; Han Zuilhof; Yves J. Chabal; Tomoki Sueyoshi; Satoshi Kera; Nobuo Ueno; Ayelet Vilan; David Cahen

doi:10.1039/C1SC00639H

Structure Matters: Correlating temperature dependent electrical transport through alkyl monolayers with vibrational and photoelectron spectroscopies†

Hagay Shpaisman,‡^a Oliver Seitz,^b Omer Yaffe,^a Katy Roodenko,^b Luc Scheres,§^c Han Zuilhof,^c Yves J. Chabal,^b Tomoki Sueyoshi,^d Satoshi Kera,^d Nobuo Ueno,^d Ayelet Vilan*^a and David Cahen*^a

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* Corresponding authors

^a Dept. of Materials & Interfaces, Weizmann Inst. of Science, Rehovot, Israel
E-mail: David.Cahen@weizmann.ac.il, Ayelet.Vilan@weizmann.ac.il

^b Dept. of Materials Science and Engineering, Univ. of Texas at Dallas, Richardson, Texas, USA

^c Lab. of Organic Chemistry, Wageningen Univ., Dreijenplein 8, Wageningen, 6703 HB, the Netherlands

^d Grad. School of Advanced Integration Science, Chiba Univ., Inage-ku, Chiba, Japan

Abstract

Freezing out of molecular motion and increased molecular tilt enhance the efficiency of electron transport through alkyl chain monolayers that are directly chemically bound to oxide-free Si. As a result, the current across such monolayers increases as the temperature decreases from room temperature to ∼80 K, i.e., opposite to thermally activated transport such as hopping or semiconductor transport. The 30-fold change for transport through an 18-carbon long alkyl monolayer is several times the resistance change for actual metals over this range. FTIR vibrational spectroscopic measurements indicate that cooling increases the packing density and reduces the motional freedom of the alkyl chains by first stretching the chains and then gradually tilting the adsorbed molecules away from the surface normal. Ultraviolet photoelectron spectroscopy shows drastic sharpening of the valence band structure as the temperature decreases, which we ascribe to decreased electron–phonon coupling. Although conformational changes are typical in soft molecular systems, in molecular electronics they are rarely observed experimentally or considered theoretically. Our findings, though, indicate that the molecular conformational changes are a prominent feature, which imply behavior that differs qualitatively from that described by models of electronic transport through inorganic mesoscopic solids.

Chemical Science

Structure Matters: Correlating temperature dependent electrical transport through alkyl monolayers with vibrational and photoelectron spectroscopies†

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Structure Matters: Correlating temperature dependent electrical transport through alkyl monolayers with vibrational and photoelectron spectroscopies

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