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Computational Studies of Electron Transport in Nanoscale Devices
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.ORCID iD: 0000-0003-3432-7012
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, a combination of density functional theory (DFT) based calculations and nonequilibrium Green’s functions are employed to investigate electron transport in molecular switches, molecular cords and nanoscale devices.

  Molecular electronic devices have been proposed as an approach to complement today’s silicon based electronic devices. However, engineering of such miniature devices and design of functional molecular components still present significant challenges.

  First, the way to connect a molecule to conductive electrodes has to be controlled. We study, in a nanoelectrode-nanoparticle platform, how structural changes affect the measured conductance and how current fluctuations due to these structural changes can be decreased. We find that, for reproducible measurements, it is important to have the molecules chemically bonded to the surfaces of adjacent nanoparticles. Furthermore, we show by a combination of DFT and theoretical modeling that we can identify signals from single-molecules in inelastic electron spectroscopy measurements on these devices.

  Second, active elements based on molecules, some examples being switches, rectifiers or memory devices, have to be designed. We study molecular conductance switches that can be operated by light and/or temperature. By tuning the substituents on the molecules, we can optimize the shift of the most conducting molecular orbital and increase the effective coupling between the molecule and the electrodes when going from the OFF to the ON-state of the switches, giving high switching ratio (up to three orders of magnitude). We also study so called mechanoswitches that are activated by a mechanical force elongating the molecules, which means that these switches could operate as sensors.

  Furthermore, we have studied two different classes of compounds that may function either as rigid molecular spacers with a well-defined conductance or as molecular cords. In both cases, we find that it is of great importance to match the conjugation of the anchoring groups with the molecular backbone for high conductance.

  The last part of the thesis is devoted to another interesting semiconductor material, diamond. We have accurately calculated the band structure and effective masses for this material. Furthermore, these results have been used to calculate the Hall coefficient, the resistivity and the Seebeck coefficient.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. , i-x, 89 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1090
Keyword [en]
Density functional theory, Molecular electronics, Organosilicon chemistry, Diamond, Molecular switches, Nanoelectrode bridge platform, Molecular cords
National Category
Condensed Matter Physics Physical Chemistry
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-209261ISBN: 978-91-554-8781-2 (print)OAI: oai:DiVA.org:uu-209261DiVA: diva2:657041
Public defence
2013-11-29, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2013-11-08 Created: 2013-10-16 Last updated: 2014-01-23
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. Identification of vibrational signatures from short chains of interlinked molecule-nanoparticle junctions obtained by inelastic electron tunnelling spectroscopy
Open this publication in new window or tab >>Identification of vibrational signatures from short chains of interlinked molecule-nanoparticle junctions obtained by inelastic electron tunnelling spectroscopy
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2013 (English)In: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 5, no 11, 4673-4677 p.Article in journal (Refereed) Published
Abstract [en]

Short chains containing a series of metal- molecule-nanoparticle nanojunctions are a nano-materials system with the potential to give electrical signatures close to those from single molecule experiments while enabling to build portable devices on a chip. Inelastic electron tunnelling spectroscopy (IETS) measurements provide one of the most characteristic electrical signals of single and few molecules. In interlinked molecule-nanoparticle (NP) chains containing of typically 5-7 molecules in a chain, the spectrum is expected to be a superposition of the vibrational signature of individual molecules. We have established a stable and reproducible molecule-AuNP multi-junction by placing few 1,8-octanedithiol (ODT) molecules into a versatile and portable nanoparticle-nanoelectrode platform and measured for the first time vibrational molecular signatures complex and coupled few-molecule-NP junctions. From quantum transport calculations, we model the IETS spectra and identify vibrational modes as well as the number of molecules contributing to the electron transport in the measured spectra.

National Category
Condensed Matter Physics Engineering and Technology
Research subject
Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-198704 (URN)10.1039/C3NR00505D (DOI)000319008700011 ()
Projects
KoF U3MEC
Available from: 2013-04-23 Created: 2013-04-23 Last updated: 2017-12-06Bibliographically approved
4. New Class of Molecular Conductance Switches Based on the [1,3]-Silyl Migration from Silanes to Silenes
Open this publication in new window or tab >>New Class of Molecular Conductance Switches Based on the [1,3]-Silyl Migration from Silanes to Silenes
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2013 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 21, 10909-10918 p.Article in journal (Refereed) Published
Abstract [en]

Based on first principles density functional theory calculations we propose a new molecularphotoswitch which exploits a photochemical [1,3]-silyl(germyl) shift leading from a silane to asilene (a Si=C double bonded compound). The silanes investigated herein act as the OFF state,with tetrahedral saturated silicon atoms disrupting the conjugation through the molecules. Thesilenes, on the other hand, have conjugated paths spanning over the complete molecules, andthus act as the ON state. We calculate ON/OFF conductance ratios in the range of 10 - 50at a voltage of +1 V. In the low bias regime the ON/OFF ratio increases to a range of 200 -1150. The reverse reaction could be triggered thermally or photolytically, with the silenebeing slightly higher in relative energy than the silane. The calculated activation barriers forthe thermal back-rearrangement of the migrating group can be tuned, and are in the range 108 -171 kJ/mol for the switches examined herein. The first principles calculations together witha simple one-level model shows that the high ON/OFF ratio in the molecule assembled in asolid state device is due to changes in the energy position of the frontier molecular orbitalscompared to the Fermi energy of the electrodes, in combination with an increased effectivecoupling between the molecule and the electrodes for the ON state.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2013
Keyword
Molecular electronics, organosilicon chemistry, electronic structure, density functional theory
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-198705 (URN)10.1021/jp400062y (DOI)000319896700005 ()
Projects
KoF U3MEC
Available from: 2013-04-23 Created: 2013-04-23 Last updated: 2017-12-06Bibliographically approved
5. Molecular Conductance Switches Exploiting Baird's Rule on Excited State Aromaticity and Antiaromaticity
Open this publication in new window or tab >>Molecular Conductance Switches Exploiting Baird's Rule on Excited State Aromaticity and Antiaromaticity
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(English)Manuscript (preprint) (Other academic)
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-209237 (URN)
Available from: 2013-10-16 Created: 2013-10-15 Last updated: 2013-11-08
6. Configuration- and Conformation-Dependent Electronic Structure Variations in 1,4-Disubstituted Cyclohexanes Enabled by a Carbon-to-Silicon Exchange
Open this publication in new window or tab >>Configuration- and Conformation-Dependent Electronic Structure Variations in 1,4-Disubstituted Cyclohexanes Enabled by a Carbon-to-Silicon Exchange
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2014 (English)In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 20, no 30, 9304-9311 p.Article in journal (Other academic) Published
Abstract [en]

Cyclohexane, with its well-defined conformers, could be an ideal force-controlled molecular switch if it were to display substantial differences in electronic and optical properties between its conformers. We utilize sigma conjugation in heavier analogues of cyclohexanes (i.e. cyclohexasilanes) and show that 1,4-disubstituted cyclohexasilanes display configuration-and conformation-dependent variations in these properties. Cis- and trans-1,4-bis(trimethylsilylethynyl)-cyclohexasilanes display a 0.11 V difference in their oxidation potentials (computed 0.11 V) and a 0.34 eV difference in their lowest UV absorption (computed difference between first excitations 0.07 eV). This is in stark contrast to differences in the corresponding properties of analogous all-carbon cyclohexanes (computed 0.02 V and 0.03 eV, respectively). Moreover, the two chair conformers of the cyclohexasilane trans isomer display large differences in electronic-structure-related properties. This enables computational design of a mechanically force-controlled conductance switch with a calculated single-molecule ON/OFF ratio of 213 at zero-bias voltage.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-209259 (URN)10.1002/chem.201402610 (DOI)000339568800023 ()
Note

De 2 sista författarna delar sistaförfattarskapet.

Available from: 2013-10-16 Created: 2013-10-16 Last updated: 2017-12-06Bibliographically approved
7. Conductance through Carbosilane Cage Compounds: A Computational Investigation
Open this publication in new window or tab >>Conductance through Carbosilane Cage Compounds: A Computational Investigation
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2013 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 42, 21692-21699 p.Article in journal (Refereed) Published
Abstract [en]

Silicon is still the dominating material in microelectronics, yet primarily π-conjugated hydrocarbons are investigated in the field of single-molecule electronics even though linear oligosilanes are σ-conjugated. A drawback with the latter is their high conformational flexibility which strongly affects conductance. Here we report on a first principles density functional theory investigation of a series of rigid [2.2.2]bicyclic carbosilanes with 3, 2, 1, or 0 disilanylene bridges, providing all-silicon paths for charge transport. It is explored if these paths can be seen as independent and equivalent current paths acting as parallel resistors. For high conductance through the carbosilanes they need to be anchored to the gold electrodes via groups that are matched with the σ-conjugated paths of the oligosilane cage segment, and we find that silyl (SiH3) groups are better matched than thiophenol groups. Even for the carbosilane with three disilanylene bridges we find that the most transmitting conductance channel is not equally distributed on the three parallel bridges. In addition, there is significant communication between the various pathways, which results in destructive interference lowering the conductance. Taken together, the different disilanylene bridges in the cage compounds do not act as parallel resistors.

Keyword
Molecular electronics, organosilicon chemistry, electronic structure, density functional theory, sigma conjugation
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-209236 (URN)10.1021/jp407485n (DOI)000326260000008 ()
Available from: 2013-10-16 Created: 2013-10-15 Last updated: 2017-12-06Bibliographically approved
8. In Search of Flexible Molecular Wires with Near Conformer-Independent Conjugation and Conductance: A Computational Study
Open this publication in new window or tab >>In Search of Flexible Molecular Wires with Near Conformer-Independent Conjugation and Conductance: A Computational Study
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2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 11, 5637-5649 p.Article in journal (Refereed) Published
Abstract [en]

Oligomers of 1,4-disila/germa/stannacyclohexa-2,5-dienes as well as all-carbon 1,4-cyclohexadienes connected via E—E single bonds (E = C, Si, Ge, or Sn) were studied through quantum chemical calculations in an effort to identify conformationally flexible molecular wires that act as molecular “electrical cords” having conformer-independent conjugative and conductive properties. Our oligomers display neutral hyperconjugative interactions (σ/π-conjugation) between adjacent σ(E—E) and π(C═C) bond orbitals, and these interactions do not change with conformation. The energies and spatial distributions of the highest occupied molecular orbitals of methyl-, silyl-, and trimethylsilyl (TMS)-substituted 1,4-disilacyclohexa-2,5-diene dimers, and stable conformers of trimers and tetramers, remain rather constant upon Si–Si bond rotation. Yet, steric congestion may be a concern in some of the oligomer types. The calculated conductances for the Si-containing tetramers are similar to that of a σ-conjugated linear all-anti oligosilane (a hexadecasilane) with equally many bonds in the conjugated paths. Moreover, the Me-substituted 1,4-disilacyclohexadiene tetramer has modest conductance fluctuations with Si–Si bond rotations when the electrode–electrode distance is locked (variation by factor 30), while the fluctuations under similar conditions are larger for the analogous TMS-substituted tetramer. When the electrode–electrode distance is changed several oligomers display small conductance variations within certain distance intervals, e.g., the mean conductance of TMS-substituted 1,4-disilacyclohexa-2,5-diene tetramer is almost unchanged over 9 Å of electrode–electrode distances.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2014
National Category
Physical Chemistry
Research subject
Chemistry with specialization in Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-209260 (URN)10.1021/jp409767r (DOI)000333381300003 ()
Available from: 2013-10-16 Created: 2013-10-16 Last updated: 2017-12-06Bibliographically approved
9. Effective masses and electronic structure of diamond including electron correlation effects in first principles calculations using the GW-approximation
Open this publication in new window or tab >>Effective masses and electronic structure of diamond including electron correlation effects in first principles calculations using the GW-approximation
2011 (English)In: AIP Advances, ISSN 2158-3226, Vol. 1, no 3, 032139- p.Article in journal (Refereed) Published
Abstract [en]

We present calculated interband transitions and effective masses for diamond from first principles including electron correlation effects via the GW-approximation. Our findings are in agreement with experiments, already the first iteration of the GW-scheme gives a direct gap at the gamma-point of 7.38 eV and a indirect gap of 5.75 eV close to experimental values. For deeper bands a quasiparticle self-consistent method is necessary to accurately reproduce the valence band width to 23.1 eV. We also obtain effective hole masses along different symmetry axes and electron conduction masses, ml = 1.1m0 and mt = 0.22m0

Place, publisher, year, edition, pages
New York: AIP, 2011
Keyword
ab initio calculations, diamond, effective mass, electron correlations, valence bands
National Category
Condensed Matter Physics Engineering and Technology
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics; Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-157835 (URN)10.1063/1.3630932 (DOI)000302139600039 ()
Available from: 2011-08-23 Created: 2011-08-23 Last updated: 2013-11-08Bibliographically approved
10. Transport coefficients in diamond from ab-initio calculations
Open this publication in new window or tab >>Transport coefficients in diamond from ab-initio calculations
2013 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 102, no 9, 092106- p.Article in journal (Refereed) Published
Abstract [en]

By combining the Boltzmann transport equation with ab-initio electronic structure calculations, we obtain transport coefficients for boron-doped diamond. We find the temperature dependence of the resistivity and the hall coefficients in good agreement with experimental measurements. Doping in the samples is treated via the rigid band approximation and scattering is treated in the relaxation time approximation. In contrast to previous results, the acoustic phonon scattering is the dominating scattering mechanism for the considered doping range. At room temperature, we find the thermopower, S, in the range 1-1.6 mV/K and the power factor, S-2 sigma, in the range 0.004-0.16 mu W/cm K-2.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2013
National Category
Condensed Matter Physics Engineering and Technology
Research subject
Physics; Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-196404 (URN)10.1063/1.4794062 (DOI)000316085200034 ()
Available from: 2013-03-08 Created: 2013-03-08 Last updated: 2017-12-06Bibliographically approved

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