Conductance of Alkanediisothiocyanates: Effect of Headgroup-Electrode Contacts
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- 作者：Ming-Dung Fu ; I-Wen Peter Chen ; Hao-Cheng Lu ; Chun-Ting Kuo ; Wei-Hsiang Tseng ; Chun-hsien Chen
- 刊名：Journal of Physical Chemistry C
- 出版年：2007
- 出版时间：August 2, 2007
- 年：2007
- 卷：111
- 期：30
- 页码：11450 - 11455
- 全文大小：154K
- 年卷期：v.111,no.30(August 2, 2007)
- ISSN：1932-7455
文摘
The realization of molecular electronics requires comprehension of single-molecule I-V characteristics. Asidefrom the electron-transport properties of the molecular framework, the molecule-electrode binding contributessignificantly to the contact resistance, Rn=0, and thus to the values of single-molecule resistance. Isothiocyanate(-NCS), a versatile ligand for organometallics, can bind to a metal substrate to complete a metal-molecule-metal configuration for external measurements. Isothiocyanate has the advantage of being a 
-conjugatedmoiety that presumably exhibits a relatively smaller impedance than the commonly used methylene thiolheadgroup (-CH2SH) in many molecular wires. For example, this study shows that the single-moleculeconductance of n-butanediisothiocyanate is an order of magnitude better than that of n-octanedithiol eventhough they both contain 10 atoms counted from sulfur to sulfur. For a homologous series of molecules, Rn=0can be extrapolated from the intercept of the resistance obtained by the repeated formation of molecularjunctions using scanning tunneling microscopy. To isolate the contact effect of the -NCS-Au electrodefrom other factors, alkanediisothiocyanates were studied because the large HOMO-LUMO gap of alkyl chainsis not sensitive to the number of methylene units. The results show two sets of Rn=0 values, with the smallerset being 128 k
, about 1/12 the other value. A detailed examination of the results suggests that the preferentialadsorption site for isothiocyanate on gold is the atop site rather than the 3-fold-hollow sites of thiol on gold.
-conjugatedmoiety that presumably exhibits a relatively smaller impedance than the commonly used methylene thiolheadgroup (-CH2SH) in many molecular wires. For example, this study shows that the single-moleculeconductance of n-butanediisothiocyanate is an order of magnitude better than that of n-octanedithiol eventhough they both contain 10 atoms counted from sulfur to sulfur. For a homologous series of molecules, Rn=0can be extrapolated from the intercept of the resistance obtained by the repeated formation of molecularjunctions using scanning tunneling microscopy. To isolate the contact effect of the -NCS-Au electrodefrom other factors, alkanediisothiocyanates were studied because the large HOMO-LUMO gap of alkyl chainsis not sensitive to the number of methylene units. The results show two sets of Rn=0 values, with the smallerset being 128 k, about 1/12 the other value. A detailed examination of the results suggests that the preferentialadsorption site for isothiocyanate on gold is the atop site rather than the 3-fold-hollow sites of thiol on gold.