Observation of the transition from tunneling to hopping carrier transport through single oligothiophene molecules
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Observation of the transition from tunneling to hopping carrier transport through single oligothiophene molecules Ryo Yamada1, Hiroaki Kumazawa1, Shoji Tanaka2, and Hirokazu Tada1 1 Graduate School of Engineering Science, Osaka University, Machikaneyama 1-3, Toyonaka, 560-8531, Japan 2 Research Center for Molecular-scale Nano Science, Institute for Molecular Science, Myodaijicho, Okazaki, Japan ABSTRACT Electrical conductance of single oligothiophene molecules with a length in the range from 2 to 9 nm was measured as a function of molecular length by a break junction method. The resistance of oligothiophenes increased exponentially from 5-mer to 14-mer while that of molecules longer than 17-mer showed linear dependence on their length. These results indicated that the carrier transport mechanism changed from tunneling to hopping around 14-mer of which the length is approximately 6 nm. INTRODUCTION The carrier transport through single molecules connected to metal electrodes has been attracted great attentions.[1] Tunneling behaviors dominate the carrier transport through the small molecules, while hopping becomes dominant in longer ones.[2-4] It is important to investigate the molecular length dependence of carrier transport to design building blocks for molecular-scale electronic devices. While the transition from tunneling to hopping was demonstrated in several works on photo-generated carrier transport measurement [5,6], there has been no report about the direct observation of the transition in electrical conductance of molecules. In this study, we have measured electrical conductance of oligothiophene wires shown in Fig.1 by a break junction method using a scanning tunneling microscope (STM). Molecules are described as (2+3m)T-di-SCN or (2+3m)-mer where m = 1~7. Oligothiophene molecules are expected to be suitable to observe the transition of the carrier transport mechanism because small energy gaps between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) are favorable for injection and tunneling of charged carriers.
(
S
NCS O
O
Si Si N S
Si Si N S S
C12 H25
)
mO
C12 H25
S
SCN O
Figure 1 Oligothiphene molecules used in this study.
EXPERIMENTAL The molecules were prepared according to the general synthetic protocols described previously.[7] The STM break junction measurement was carried out at room temperature by using mechanically cut gold tips on Au(111) surfaces in an approximately 1 mM toluene solution of the molecules. The electrical conductance of molecules was measured by the STM break junction method in which a tip of STM is repeatedly brought into and out of contact with a substrate in a solution of molecules.[8] When the tip is pulled up after the contact with the substrate, the measured conductance changes in a stepwise manner at integer multiples of quantum conductance G0 = 2e2 / h (= 77.4 µS) due to the formation of atomic contact of metals. After the atomic contact of metals is broken, a new sequence of conductance steps is
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