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Vibrational signatures in inelastic tunneling spectroscopy from short molecule-nanoparticle chains trapped in versatile nanoelectrodes
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Experimentell fysik. (Electron Microscopy and Nanoenginnering)
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Experimentell fysik. (Electron Microscopy and Nanoenginnering)
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Teoretisk fysik.
University Basel, Department of Physics, CH-4056 Basel, Switzerland.
Vise andre og tillknytning
(engelsk)Manuskript (preprint) (Annet vitenskapelig)
Emneord [en]
Molecular vibrations, Inelastic electron tunneling spectroscopy, nanoparticle-nanoelectrode bridge platform, octandithiol
HSV kategori
Forskningsprogram
Teknisk fysik med inriktning mot materialanalys; Fysik och astronomi med inriktning mot teoretisk fysik
Identifikatorer
URN: urn:nbn:se:uu:diva-160626OAI: oai:DiVA.org:uu-160626DiVA, id: diva2:451959
Tilgjengelig fra: 2011-10-27 Laget: 2011-10-27 Sist oppdatert: 2012-04-01
Inngår i avhandling
1. Building Systems for Electronic Probing of Single Low Dimensional Nano-objects: Application to Molecular Electronics and Defect Induced Graphene
Åpne denne publikasjonen i ny fane eller vindu >>Building Systems for Electronic Probing of Single Low Dimensional Nano-objects: Application to Molecular Electronics and Defect Induced Graphene
2011 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

Nano-objects have unique properties due to their sizes, shapes and structure. When electronic properties of such nano-objects are used to build devices, the control of interfaces at atomic level is required.

In this thesis, systems were built that can not only electrically characterize nano-objects, but also allow to analyze a large number of individual nano-objects statistically at the example of graphene and nanoparticle-molecule-nanoelectrode junctions.

An in-situ electrical characterization system was developed for the analysis of free standing graphene sheets containing defects created by an acid treatment. The electrical characterization of several hundred sheets revealed that the resistance in acid treated graphene sheets decreased by 50 times as compared to pristine graphene and is explained by the presence of di-vacancy defects. However, the mechanism of defect insertion into graphene is different when graphene is bombarded with a focused ion beam and in this case, the resistance of graphene increases upon defect insertion. The defect insertion becomes even stronger at liquid N2 temperature.

A molecular electronics platform with excellent junction properties was fabricated where nanoparticle-molecule chains bridge 15-30nm nanoelectrodes. This approach enabled a systematic evaluation of junctions that were assembled by functionalizing electrode surfaces with alkanethiols and biphenyldithiol. The variations in the molecular device resistance were several orders of magnitude and explained by variations in attachment geometries of molecules. 

The spread of resistance values of different devices was drastically reduced by using a new functionalization technique that relies on coating of gold nanoparticles with trityl protected alkanedithiols, where the trityl group was removed after trapping of nanoparticles in the electrode gap. This establishment of a reproducible molecular electronics platform enabled the observation of vibrations of a few molecules by inelastic tunneling spectroscopy. Thus this system can be used extensively to characterize molecules as well as build devices based on molecules and nanoparticles. 

sted, utgiver, år, opplag, sider
Uppsala: Acta Universitatis Upsaliensis, 2011. s. 109
Serie
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 877
Emneord
Graphene, defect induced graphene, molecular electronics, nanoelectrodes, nanoparticles, conductivity, junction, nanomaterial, focused ion beam, surface functionalization, electrical characterization
HSV kategori
Forskningsprogram
Teknisk fysik med inriktning mot materialanalys
Identifikatorer
urn:nbn:se:uu:diva-160630 (URN)978-91-554-8212-1 (ISBN)
Disputas
2011-12-12, Häggsalen, Ångströmlab, Lägerhyddsvägen 1, Uppsala, 10:15 (engelsk)
Opponent
Veileder
Tilgjengelig fra: 2011-11-21 Laget: 2011-10-27 Sist oppdatert: 2011-11-23bibliografisk kontrollert
2. Theoretical studies of a nanoparticle bridge platform for molecular electronics measurements
Åpne denne publikasjonen i ny fane eller vindu >>Theoretical studies of a nanoparticle bridge platform for molecular electronics measurements
2011 (engelsk)Licentiatavhandling, med artikler (Annet vitenskapelig)
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. 

sted, utgiver, år, opplag, sider
Uppsala: Department of Physics and Astronomy, 2011. s. 31
HSV kategori
Forskningsprogram
Fysik med inriktning mot atom- molekyl- och kondenserande materiens fysik
Identifikatorer
urn:nbn:se:uu:diva-162061 (URN)
Presentation
2011-11-24, Oseenska rummet, Lägerhyddsvägen 1, Uppsala, 10:59 (engelsk)
Opponent
Veileder
Tilgjengelig fra: 2011-11-23 Laget: 2011-11-23 Sist oppdatert: 2011-11-24bibliografisk kontrollert

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