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Theoretical studies of a nanoparticle bridge platform for molecular electronics measurements
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The main focus of this thesis is the theoretical investigations of a nanogap platform used for molecular electronics measurements under ambient conditions. The nanogap is about 20 nm wide, while the molecules investigated here (octanethiol(OT) and octanedithiol(ODT)) are about 1-1.5 nm long making it impossible to bridge the gap with one molecule. Two different approaches are investigated. In the first approach the electrodes of the nanogap are coated with a layer of OT molecules, and large gold nanoparticles (diameter of about 30 nm) are trapped in the gap creating two molecular junctions with assemblies of molecules. In the second approach the electrodes are kept clean, but instead the gold nanoparticles are coated with doubly functionalized molecules (ODT) and trapped in the gap. Here the nanoparticles are limited in size to about 5 nm, hence it is necessary to consider nanoparticle-molecule chains or small networks to bridge the gap. The first principles modeling of the structure of the metal-molecule junctions combined with elastic and inelastic transport properties is performed using the density functional theory (DFT) combined with the non-equilibrium Green’s functions (DFT-NEGF) method.

In the first approach with the coated electrodes and the large nanoparticles, simulations show that structural irregularities at the electrode interface can lead to a significant variation of the conductance through the molecular film. Due to the size of the nanoparticles, the shape and orientation of the facets will have great influence on how many molecules are connected, affecting the measured resistance of the device.

With the second approach utilizing the functionalized nanoparticles, more stable junctions are obtained since the nanogap is bridged by molecular junctions chemisorbed in both ends. To make chemical bonds to both sides of the junctions, the outer functional group needs to be protected before the trapping of nanoparticles in the gap. Deprotected nanoparticles agglomerate and cannot be trapped. We have inves- tigated the most probable configurations of the molecules in these junctions. During deprotection of the functional group in the gap, a conduction increase have been observed. We have found that the removal of the protection group is not responsible for the increased conduction. Instead, since the deprotected molecule is shorter and the nanoparticles are mobile during deprotection, a reorganization of the nanopar- ticles in the gap occurs. This reorganization leads to decreasing of the tunneling length for the electrons, hence increasing the conduction.

We also demonstrate, that we can obtain the inelastic electron tunneling spectroscopy (IETS) signature of an octanedithiol molecule in this platform. This is done on the network of chemisorbed ODT junctions, where we are able to relate the low-bias Au-S and C-S stretch modes of the molecule to observed peaks in IETS. From this we estimate that the main contribution in the signal comes from chains containing 5, 6 and 7 molecular junctions. To identify the peaks, we have calculated the theoretical spectra for one molecule, from which we are able to extract the important vibrational modes, and their couplings to the electrons. This we then use in a model, including the Coulomb blockade observed in the nanoparticles, to fit the theoretical spectra to the measured one. 

Place, publisher, year, edition, pages
Uppsala: Department of Physics and Astronomy , 2011. , 31 p.
National Category
Condensed Matter Physics Nano Technology
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-162061OAI: oai:DiVA.org:uu-162061DiVA: diva2:458529
Presentation
2011-11-24, Oseenska rummet, Lägerhyddsvägen 1, Uppsala, 10:59 (English)
Opponent
Supervisors
Available from: 2011-11-23 Created: 2011-11-23 Last updated: 2011-11-24Bibliographically approved
List of papers
1. Assessment of a nanoparticle bridge platform for molecular electronics measurements
Open this publication in new window or tab >>Assessment of a nanoparticle bridge platform for molecular electronics measurements
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2010 (English)In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 21, no 43, 435204- p.Article in journal (Refereed) Published
Abstract [en]

A combination of electron beam lithography, photolithography and focused ion beam milling was used to create a nanogap platform, which was bridged by gold nanoparticles in order to make electrical measurements and assess the platform under ambient conditions. Non-functionalized electrodes were tested to determine the intrinsic response of the platform and it was found that creating devices in ambient conditions requires careful cleaning and awareness of the contributions contaminants may make to measurements. The platform was then used to make measurements on octanethiol (OT) and biphenyldithiol (BPDT) molecules by functionalizing the nanoelectrodes with the molecules prior to bridging the nanogap with nanoparticles. Measurements on OT show that it is possible to make measurements on relatively small numbers of molecules, but that a large variation in response can be expected when one of the metal–molecule junctions is physisorbed, which was partially explained by attachment of OT molecules to different sites on the surface of the Au electrode using a density functional theory calculation. On the other hand, when dealing with BPDT, high yields for device creation are very difficult to achieve under ambient conditions. Significant hysteresis in the IV curves of BPDT was also observed, which was attributed primarily to voltage induced changes at the interface between the molecule and the metal.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-132279 (URN)10.1088/0957-4484/21/43/435204 (DOI)000282511100005 ()
Projects
KoF U3MEC
Available from: 2010-10-18 Created: 2010-10-18 Last updated: 2017-12-12Bibliographically approved
2. Realization of highly reproducible molecular junctions in a nanoparticle-alkanedithiol-nanoelectrode bridge platform
Open this publication in new window or tab >>Realization of highly reproducible molecular junctions in a nanoparticle-alkanedithiol-nanoelectrode bridge platform
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(English)Manuscript (preprint) (Other academic)
Keyword
Molecular electronic devices, electrical characterization, alkanedithiols, chemisorbed molecular junctions, nanoparticle-nanoelectrode
National Category
Nano Technology Other Electrical Engineering, Electronic Engineering, Information Engineering Atom and Molecular Physics and Optics
Research subject
Engineering Science with specialization in Materials Analysis; Physics and Astronomy specializing in Theoretical Physics
Identifiers
urn:nbn:se:uu:diva-160622 (URN)
Projects
KoF U3MEC
Available from: 2011-10-27 Created: 2011-10-27 Last updated: 2013-11-08
3. Vibrational signatures in inelastic tunneling spectroscopy from short molecule-nanoparticle chains trapped in versatile nanoelectrodes
Open this publication in new window or tab >>Vibrational signatures in inelastic tunneling spectroscopy from short molecule-nanoparticle chains trapped in versatile nanoelectrodes
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(English)Manuscript (preprint) (Other academic)
Keyword
Molecular vibrations, Inelastic electron tunneling spectroscopy, nanoparticle-nanoelectrode bridge platform, octandithiol
National Category
Nano Technology Other Electrical Engineering, Electronic Engineering, Information Engineering Atom and Molecular Physics and Optics
Research subject
Engineering Science with specialization in Materials Analysis; Physics and Astronomy specializing in Theoretical Physics
Identifiers
urn:nbn:se:uu:diva-160626 (URN)
Available from: 2011-10-27 Created: 2011-10-27 Last updated: 2012-04-01

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