| 1. |
- Emanuelsson, Rikard, et al.
(författare)
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Configuration- and Conformation-Dependent Electronic-Structure Variations in 1,4-Disubstituted Cyclohexanes Enabled by a Carbon-to-Silicon Exchange
- 2014
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 20:30, s. 9304-9311
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Emanuelsson, Rikard, et al.
(författare)
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In Search of Flexible Molecular Wires with Near Conformer-Independent Conjugation and Conductance : A Computational Study
- 2014
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:11, s. 5637-5649
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Tidskriftsartikel (refereegranskat)abstract
- 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" hang conformer-independent conjugative and conductive properties. Our oligomers display neutral hyperconjugative interactions (sigma/pi-conjugation) between adjacent sigma(E-E) and pi(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 sigma-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 similar to 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 A of electrode-electrode distances.
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| 3. |
- Jafri, Hassan M., et al.
(författare)
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Identification of vibrational signatures from short chains of interlinked molecule-nanoparticle junctions obtained by inelastic electron tunnelling spectroscopy
- 2013
-
Ingår i: Nanoscale. - 2040-3364 .- 2040-3372. ; 5:11, s. 4673-4677
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 4. |
- Jafri, S. Hassan M., et al.
(författare)
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Nano-fabrication of molecular electronic junctions by targeted modification of metal-molecule bonds
- 2015
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Ingår i: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 5
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Tidskriftsartikel (refereegranskat)abstract
- Reproducibility, stability and the coupling between electrical and molecular properties are central challenges in the field of molecular electronics. The field not only needs devices that fulfill these criteria but they also need to be up-scalable to application size. In this work, few-molecule based electronics devices with reproducible electrical characteristics are demonstrated. Our previously reported 5 nm gold nanoparticles (AuNP) coated with omega-triphenylmethyl (trityl) protected 1,8-octanedithiol molecules are trapped in between sub-20 nm gap spacing gold nanoelectrodes forming AuNP-molecule network. When the trityl groups are removed, reproducible devices and stable Au-thiol junctions are established on both ends of the alkane segment. The resistance of more than 50 devices is reduced by orders of magnitude as well as a reduction of the spread in the resistance histogram is observed. By density functional theory calculations the orders of magnitude decrease in resistance can be explained and supported by TEM observations thus indicating that the resistance changes and strongly improved resistance spread are related to the establishment of reproducible and stable metal-molecule bonds. The same experimental sequence is carried out using 1,6-hexanedithiol functionalized AuNPs. The average resistances as a function of molecular length, demonstrated herein, are comparable to the one found in single molecule devices.
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| 5. |
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| 6. |
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| 7. |
- Löfås, Henrik, 1983-, et al.
(författare)
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A computational study of potential molecular switches that exploit Baird's rule on excited-state aromaticity and antiaromaticity
- 2014
-
Ingår i: Faraday discussions. - 1359-6640 .- 1364-5498. ; 174, s. 105-124
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 8. |
- Löfås, Henrik, et al.
(författare)
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Conductance through Carbosilane Cage Compounds : A Computational Investigation
- 2013
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:42, s. 21692-21699
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Tidskriftsartikel (refereegranskat)abstract
- Silicon is still the dominating material in microelectronics, yet primarily conjugated hydrocarbons are investigated in the field of single-molecule electronics even though linear oligosilanes are a-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 a-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.
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| 9. |
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| 10. |
- Löfås, Henrik, et al.
(författare)
-
New class of molecular conductance switches based on the [1,3]-silyl migration from silanes to silenes
- 2013
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:21, s. 10909-10918
-
Tidskriftsartikel (refereegranskat)abstract
- On the basis of first-principles density functional theory calculations, we propose a new molecular photoswitch which exploits a photochemical [1,3]-silyl(germyl) shift leading from a silane to a silene (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. The silenes, on the other hand, have conjugated paths spanning over the complete molecules and thus act as the ON state. We calculate ON/OFF conductance ratios in the range of 10-50 at 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 silene being slightly higher in relative energy than the silane. The calculated activation barriers for the 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 with a simple one-level model show that the high ON/OFF ratio in the molecule assembled in a solid state device is due to changes in the energy position of the frontier molecular orbitals compared to the Fermi energy of the electrodes, in combination with an increased effective coupling between the molecule and the electrodes for the ON state.
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| 11. |
- Löfås, Henrik, et al.
(författare)
-
The [1,3]-Si→O Silyl Shift from a Nonconducting Acylsilane to a Conducting Brook-Silene as Basis for a Molecular Switch
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- By usage of density functional theory (DFT) calculations we explored if the [1,3]-silyl shift leading from an acylsilane with two p-conjugated substituents to a silene (a Si=C double bonded compound) can be used as a basis for a molecular conductance switch. In such a switch, the acylsilane, with a tetrahedral saturated silicon atom disrupting the conjugation through the molecule, acts as the OFF state, whereas the silene with a conjugated path running through the complete molecule represents the ON state. Our requirements are (i) the silenes should be slightly higher in relative energy than the acylsilane so as to promote a thermal backrearragment, (ii) the barrier for the backtransfer of the silyl group should be 25-30 kcal/mol, (iii) the ON/OFF conductance ratio should be high, and (iv) the switch should be realistic. According to our calculations using non-equilibrium Green’s function theory, a 1,2-bis(4-thiophenylethynyl)silene has a conductance which is 270 times higher than that of the corresponding acylsilane at zero bias voltage. However, at a voltage of +1 V the ON/OFF ratio decreases to ~40.
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| 12. |
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| 13. |
- Jafri, S. Hassan M., et al.
(författare)
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Assessment of a nanoparticle bridge platform for molecular electronics measurements
- 2010
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 21:43, s. 435204-
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Tidskriftsartikel (refereegranskat)abstract
- 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 I-V curves of BPDT was also observed, which was attributed primarily to voltage induced changes at the interface between the molecule and the metal.
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| 14. |
- Jafri, S.Hassan M., et al.
(författare)
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Control of junction resistances in molecular electronic devices fabricated by FIB
- 2010
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Ingår i: 36th International Conference on Micro and Nano Engineering, MNE2010, Italy (2010).
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Konferensbidrag (refereegranskat)abstract
- Molecules provide an opportunity to fabricate electronic devices with much smaller basic unit in size i.e. 1-5 nm as compared to today’s silicon based electronics. Furthermore, molecules can be synthesized withalmost unlimited variation of their electronic structure. Theoretically, molecules in various configurations were demonstrated as rectifiers, transistors or memories, but experimentally it is still very difficult to obtaina stable and reproducible molecular based device [1]. A major hurdle to realize such devices is to make reliable electrical contacts to a single or a few molecules. Here, we show the first reproducible and systematic evaluation of a nanogap-nanoparticle bridge set-up that can be used as base for development of few molecule molecular electronics under ambient conditions. We have developed a nano-contact platform by top-down approach [2] with a gap size of 20-30nm using combined techniques of photolithography, electron beam lithography and focused ion beam milling (Fig 1). These gaps demonstrate excellent resistance in order of 1000 TΩ enabling us to carry out electrical characterization of highly resistive nanomaterials.However, compared to the size of molecules these gaps are quite big. In this study, we used metallic nanoparticles to bridge the gap and thus obtain electrical contacts with 1-2nm long molecules in the junction between the nanoelectrodes and the nanoparticles. The nanoparticles are assembled in the gap by a bottom-up approach using dielectrophrosis trapping process. Prior to introduction of molecules in such devices, we found that the trapping of gold nanoparticles (AuNP) in between clean nanoelectrodes without presence of molecules often gave resistance in order of mega-ohms to giga-ohms due to presence of a non conductive barrier. However, it was observed that cleaning protocols of both the gold contacts and nanoparticles in solution lead to resistance of less than a few hundreds of ohms (Fig 2). Molecules were introduced both by functionalizing the electrode gap and the the nanoparticles and the results of both functionalisation protocols are compared. By optimizing the electrode cleaning as well as the functionalisation of the metallic surfaces, we obtain reproducible electrical measurements. We fabricated such devices either by depositing a Self Assembled Monolayer (SAM) of molecules on the nano-contacts and bridging the gap by AuNP or by bridging the clean nano-contacts with molecule-coated-AuNP (Fig 3). Here we utilized a model molecules octanethiol (OT), octanedithiol and biphenyldithiol in fabrication of devices and study of metal molecule junction resistance. IV characterization of OT molecules (Fig 4) showed linear response where current levels varied between picoamps and femtoamps with an applied voltage of 1-3V. OT in this setup had one physisorbed contact with gold, which resulted in much less wave function mixing at the molecule-metal interface, and consequently decreased the transmission probability at the molecule-electrode interface. As a result, in the evaluation of more than 50 devices, a considerable variation of resistance between different devices due to the lack of covalent binding, the variation in number of trapped AuNPs, incomplete coverage of OT on the uneven surface of nanoelectrodes and variation in contact surface geometry. Density functional theory is used to understand the origin of the resistance fluctuation. We were able to estimate the average resistance per octanethiol molecule for such device in order of 175GΩ, in good agreement with other published results. Our results with the measurements on OT in such devices demonstrate that it is possible to fabricate stable electronic devices having relatively small numbers of molecules with reliable metal molecule junction by combing top-down and bottom-up approaches. By functionalizing the nanoparticles, we obtained a strong decrease of the resistance spread of such devices from 3 orders of magnitude to about 1 order of magnitude, making this technology a potential approach for molecular devices operating at ambient conditions.
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| 15. |
- Kaewmaraya, Thanayut, et al.
(författare)
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Hybrid density functional study of electronic and optical properties of phase change memory material : Ge2Sb2Te5
- 2013
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 113:3, s. 033510-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 16. |
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| 17. |
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| 18. |
- Löfås, Henrik, 1983-
(författare)
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Computational Studies of Electron Transport in Nanoscale Devices
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 19. |
- Löfås, Henrik, et al.
(författare)
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Computational Study of the Chaotic Behavior in Single-molecule Conduction
- 2013
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Ingår i: 2013 MRS Spring Meeting.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- 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).
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| 20. |
- Löfås, Henrik, 1983-, et al.
(författare)
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Effective masses and electronic structure of diamond including electron correlation effects in first principles calculations using the GW-approximation
- 2011
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Ingår i: AIP Advances. - New York : AIP. - 2158-3226. ; 1:3, s. 032139-
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Tidskriftsartikel (refereegranskat)abstract
- 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
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| 21. |
- Löfås, Henrik, et al.
(författare)
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F-region electron heating by X-mode radiowaves in underdense conditions
- 2009
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 27:6, s. 2585-2592
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Tidskriftsartikel (refereegranskat)abstract
- Observations of modifications of the electron temperature in the F-region produced by powerful high frequency radiowaves at 4.04 MHz transmitted in X-mode are presented. The experiments were performed during quiet nighttime conditions with low ionospheric densities so no reflections occurred. Electron temperature enhancements of the order of 300-400K were obtained. Numerical simulation of ohmic heating by the pump wave reproduces both altitude profiles and temporal dependence of the temperature modifications in the experiments.
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| 22. |
- Löfås, Henrik, 1983-
(författare)
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Theoretical studies of a nanoparticle bridge platform for molecular electronics measurements
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 23. |
- Löfås, Henrik, 1983-, et al.
(författare)
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Transport coefficients in diamond from ab-initio calculations
- 2013
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 102:9, s. 092106-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 24. |
- Pathak, Biswarup, et al.
(författare)
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Double-functionalized nanopore-embedded gold electrodes for rapid DNA sequencing
- 2012
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 100:2, s. 023701-
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the effect of double-functionalization on gold electrodes for improving nanopore-based DNA sequencing. The functionalizing molecular probes are, respectively, capable of temporarily forming hydrogen bonds with both the nucleobase part and the phosphate group of the target DNA, thus potentially minimizing the structural fluctuations of a single-stranded DNA molecule passing between the gold electrodes. The results of our first-principles study indicate that the proposed setup yields current signals that differ by at least 1 order of magnitude for the four different nucleic acid bases, thus offering the possibility to electrically distinguish them.
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