Optical and Electrical Properties of Three-Dimensional Interlinked Gold Nanoparticle Assemblies

Abstract

The optical and electrical properties of 11-20 nm thick films composed of approximately 4 nm gold nanoparticles (Au-NPs) interlinked by six organic dithiol or bis-dithiocarbamate derivatives were compared to investigate how these properties depend on the core of the linker molecule (benzene or cyclohexane) and its metal-binding substituents (thiol or dithiocarbamate). Films prepared with the thiol-terminated linker molecules, (1,4-bis(mercaptomethyl)benzene, 1,4-bis(mercaptomethyl)cyclohexane, 1,4-bis(mercaptoacetamido)benzene, and 1,4-bis(mercaptoacetamido)cyclohexane), exhibit thermally activated charge transport. The activation energies lie between 59 and 71 meV. These films show distinct plasmon absorption bands with maxima between 554 and 589 nm. In contrast, the film prepared with 1,4-cyclohexane-bis(dithiocarbamate) has a significantly red-shifted plasmon band ( approximately 626 nm) and a pronounced absorbance in the near infrared. The activation energy for charge transport is only 14 meV. These differences are explained in terms of the formation of a resonant state at the interface due to overlap of the molecular orbital and metal wave function, leading to an apparent increase in NP diameter. The film prepared with 1,4-phenylene-bis(dithiocarbamate) exhibits metallic properties, indicating the full extension of the electron wave function between interlinked NPs. In all cases, the replacement of the benzene ring with a cyclohexane ring in the center of the linker molecule leads to a 1 order of magnitude decrease in conductivity. A linear relationship is obtained when the logarithm of conductivity is plotted as a function of the number of nonconjugated bonds in the linker molecules. This suggests that nonresonant tunneling along the nonconjugated parts of the molecule governs the electron tunneling decay constant (beta(N)(-)(CON)), while the contribution from the conjugated parts of the molecule is weak (corresponding to resonant tunneling). The obtained value for beta(N)(-)(CON) is approximately 1.0 (per non-conjugated bond) and independent of the nanoparticle-binding group. Hence, the molecules can be viewed as consisting of serial connections of electrically insulating (nonconjugated) and conductive (conjugated) parts.

Gold in a Metallic Divided State—From Faraday to Present‐Day Nanoscience

“…Of greatest interest is the potential to precisely tune both the optical and electronic properties of the films by controlling both the individual particle size and the interparticle spacing, the latter now with molecular-scale precision through the choice of appropriate protecting or capping molecules surrounding the gold particle. [49,50] In a recent timely contribution Pelka et al reported extensive studies on selfassembled thin films of gold nanoparticles of 4-5 nm diameter, prepared on glass, using aliphatic dithiols of different hydrocarbon chain lengths as interparticle linker molecules. [51] Importantly, the dc conductivity of these films (s), measured down to 4.2 K, shows a strong dependence on the intervening spacer length, effectively traversing the different mechanisms of electrical conductivity through the films: from thermally activated electron hopping between gold particles in films with longer-chain linker molecules, changing to electron tunnelling at low temperatures, ultimately to become metallic when the interparticle linker is shorter (Figure 9).…”

Anderson Insulators in Self-Assembled Gold Nanoparticles Thin Films: Single Electron Hopping between Charge Puddles Originated from Disorder

“…Some experiments demonstrate the possibility of control coupling strength by means other than controlling the length the molecule, such as NP coverage and film thickness. [13,14] Though successfully explaining the observed transition, the above picture is based on the regularity of the NP assembly, in which the randomness cannot simply be neglected. There are at least three types of randomness: NP position and size, interparticle coupling, and random offset charges.…”

Electron transport in a gold nanoparticle assembly structure stabilized by a physisorbed porphyrin derivative

“…The latter results suggest that the molecular orbitals of the linking molecules play an important role in the electron transport properties of the gold nanoparticle assemblies. Wessels et al 6 have studied the conductivity of gold nanoparticle assemblies from the viewpoint of the conjugation strength of the molecules. They found that the conductivity exhibited an exponential decay versus the number of nonconjugated bonds in the linker molecule, suggesting that these molecules can be viewed as being serial connections of electrically insulating ͑nonconju-gated͒ and conductive ͑conjugated͒ parts.…”