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Löfås, H., Jahn, B. O., Wärnå, J., Emanuelsson, R., Ahuja, R., Grigoriev, A. & Ottosson, H. (2014). A computational study of potential molecular switches that exploit Baird's rule on excited-state aromaticity and antiaromaticity. Faraday discussions (Online), 174, 105-124
Open this publication in new window or tab >>A computational study of potential molecular switches that exploit Baird's rule on excited-state aromaticity and antiaromaticity
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2014 (English)In: Faraday discussions (Online), ISSN 1359-6640, E-ISSN 1364-5498, Vol. 174, p. 105-124Article in journal (Refereed) Published
Abstract [en]

A series of tentative single-molecule conductance switches which could be triggered by light were examined by computational means using density functional theory (DFT) with non-equilibrium Green's functions (NEGF). The switches exploit the reversal in electron counting rules for aromaticity and antiaromaticity upon excitation from the electronic ground state (S0) to the lowest [small pi][small pi]* excited singlet and triplet states (S1 or T1), as described by Huckel's and Baird's rules, respectively. Four different switches and one antifuse were designed which rely on various photoreactions that either lead from the OFF to the ON states (switches 1, 2 and 4, and antifuse 5) or from the ON to the OFF state (switch 3). The highest and lowest ideal calculated switching ratios are 1175 and 5, respectively, observed for switches 1 and 4. Increased thermal stability of the 1-ON isomer is achieved by benzannulation (switch 1B-OFF/ON). The effects of constrained electrode-electrode distances on activation energies for thermal hydrogen back-transfer from 1-ON to 1-OFF and the relative energies of 1-ON and 1-OFF at constrained geometries were also studied. The switching ratio is strongly distance-dependent as revealed for 1B-ON/OFF where it equals 711 and 148 when the ON and OFF isomers are calculated in electrode gaps with distances confined to either that of the OFF isomer or to that of the ON isomer, respectively.

National Category
Organic Chemistry
Research subject
Chemistry with specialization in Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-238681 (URN)10.1039/C4FD00084F (DOI)000348331600007 ()
Available from: 2014-12-15 Created: 2014-12-15 Last updated: 2017-12-05Bibliographically approved
Emanuelsson, R., Löfås, H., Wallner, A., Nauroozi, D., Baumgartner, J., Marschner, C., . . . Ottosson, H. (2014). Configuration- and Conformation-Dependent Electronic Structure Variations in 1,4-Disubstituted Cyclohexanes Enabled by a Carbon-to-Silicon Exchange. Chemistry - A European Journal, 20(30), 9304-9311
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, p. 9304-9311Article 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
Emanuelsson, R., Löfås, H., Zhu, J., Ahuja, R., Grigoriev, A. & Ottosson, H. (2014). In Search of Flexible Molecular Wires with Near Conformer-Independent Conjugation and Conductance: A Computational Study. The Journal of Physical Chemistry C, 118(11), 5637-5649
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, p. 5637-5649Article 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
Löfås, H. (2013). Computational Studies of Electron Transport in Nanoscale Devices. (Doctoral dissertation). Uppsala: Acta Universitatis Upsaliensis
Open this publication in new window or tab >>Computational Studies of Electron Transport in Nanoscale Devices
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. p. i-x, 89
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1090
Keywords
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:nbn:se:uu:diva-209261 (URN)978-91-554-8781-2 (ISBN)
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
Löfås, H., Grigoriev, A., Isberg, J. & Ahuja, R. (2013). Computational Study of the Chaotic Behavior in Single-molecule Conduction. In: 2013 MRS Spring Meeting: Electrical Contacts to Nanomaterials and Nanodevices. Paper presented at 2013 MRS Spring Meeting.
Open this publication in new window or tab >>Computational Study of the Chaotic Behavior in Single-molecule Conduction
2013 (English)In: 2013 MRS Spring Meeting: Electrical Contacts to Nanomaterials and Nanodevices, 2013Conference paper, Poster (with or without abstract) (Other academic)
Abstract [en]

Recently we have seen great advances in synthesis and fabrication of nanostructures. However, there is still no consensus on the conductance of small organic molecules, where different values of the conductance are often attributed to differences in metal-molecule interface structure or different molecular conformations[1,2]. Control and characterization of the metal-molecule interface during formation of the junction is in practice an impossible task. To get insight into this highly dynamic process, computer simulations are needed; here we are going to show a combination of ab-initio molecular dynamics (MD)-simulations and conductance calculations to address this problem.The conductance of a junction is mainly determined by the relative position of the energy level closest to the Fermi level of the electrodes and by the coupling of the corresponding electronic state to the electrodes[2]. These parameters are greatly influenced by the nature of the interaction and/or chemical bond between electrodes and the molecule. Information about the nature of this interaction and its variation with different binding sites can be extracted from the conduction spectra. Here we are using MD-simulations to get an unbiased set of geometries, thus mimicking the randomness of a real junction under thermal fluctuations. From the obtained geometries the zero-bias conductance is calculated and used for histograms to investigate the statistics of the junction.The obtained histograms for the thiol-bonded molecules are fitted with probability distributions for different Gaussian ensembles and we show that the interaction between the electrode and the molecule gives rise to quantum chaos in the junction. The effect of quantum chaos have earlier been found experimentally for quantum dots[3] and nanowires[4]. By removing the symmetry in the junction the chaotic behavior can be increased. We also compare the thiol anchoring groups with amines and we can see that the weaker coupling to the gold for the amines increases the conductance fluctuations in the junctions by one to two orders of magnitude. By tuning the ratio of the coupling between the electrodes and the molecular state we show, that the junction can be switched from a chaotic behavior to a case with a normal distributed conductance spectrum where only temperature fluctuations are present.[1] S. L. Bernasek, Angew. Chem. Int. Ed. 51, 9737 (2012).[2] A. Nitzan and M. A. Ratner, Science 300, 1384 (2003).[3] L. A. Ponomarenko, F. Schedin, M. I. Katsnelson, R. Yang, E. W. Hill, K. S. Novoselov, and A. K. Geim, Science 320, 356 (2008).[4] J. L. Costa-Krämer, N. García, P. García-Mochales, P. A. Serena, M. I. Marqués, and A. Correia, Phys. Rev. B 55, 5416 (1997).

National Category
Other Physics Topics Electrical Engineering, Electronic Engineering, Information Engineering
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-198703 (URN)
Conference
2013 MRS Spring Meeting
Projects
KoF U3MEC
Available from: 2013-04-23 Created: 2013-04-23 Last updated: 2016-04-22
Löfås, H., Emanuelsson, R., Ahuja, R., Grigoriev, A. & Ottosson, H. (2013). Conductance through Carbosilane Cage Compounds: A Computational Investigation. The Journal of Physical Chemistry C, 117(42), 21692-21699
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, p. 21692-21699Article 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.

Keywords
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
Kaewmaraya, T., Ramzan, M., Löfås, H. & Ahuja, R. (2013). Hybrid density functional study of electronic and optical properties of phase change memory material: Ge2Sb2Te5. Journal of Applied Physics, 113(3), 033510
Open this publication in new window or tab >>Hybrid density functional study of electronic and optical properties of phase change memory material: Ge2Sb2Te5
2013 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 113, no 3, p. 033510-Article in journal (Refereed) Published
Abstract [en]

In this article, we use hybrid density functional (HSE06) to study the crystal and electronic structures and optical properties of well known phase change memory material Ge2Sb2Te5. We calculate the structural parameters, band gaps, and dielectric functions of three stable structures of this material. We also analyze the electron charge distribution using the Bader's theory of charge analysis. We find that hybrid density functional slightly overestimates the value of "c" parameter. However, overall, our results calculated with the use of hybrid density functional (HSE06) are very close to available experimental values than calculated with the use of Perdew Burke-Ernzerhof functional. Specifically, the electronic band gap values of this material calculated with HSE06 are in good agreement with the available experimental data in the literature. Furthermore, we perform the charge analysis and find that naive ionic model fails to explain the charge distribution between the constituent atoms, showing the complex nature of this compound.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-195377 (URN)10.1063/1.4775715 (DOI)000313670600020 ()
Available from: 2013-02-25 Created: 2013-02-25 Last updated: 2017-12-06Bibliographically approved
Jafri, H. M., Löfås, H., Jonas, F., Blom, T., Grigoriev, A., Wallner, A., . . . Leifer, K. (2013). Identification of vibrational signatures from short chains of interlinked molecule-nanoparticle junctions obtained by inelastic electron tunnelling spectroscopy. Nanoscale, 5(11), 4673-4677
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, p. 4673-4677Article 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: 2019-04-24Bibliographically approved
Löfås, H., Orthaber, A., Jahn, B. O., Rouf, A. M., Grigoriev, A., Ott, S., . . . Ottosson, H. (2013). New Class of Molecular Conductance Switches Based on the [1,3]-Silyl Migration from Silanes to Silenes. The Journal of Physical Chemistry C, 117(21), 10909-10918
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, p. 10909-10918Article 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
Keywords
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
Löfås, H., Grigoriev, A., Isberg, J. & Ahuja, R. (2013). Transport coefficients in diamond from ab-initio calculations. Applied Physics Letters, 102(9), 092106
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, p. 092106-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
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-3432-7012

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