Electrical conduction through single molecules and self-assembled monolayers

Akkerman, Hylke B.; Boer, Bart de

doi:10.1088/0953-8984/20/01/013001

Cited by 363 publications

(571 citation statements)

References 168 publications

(343 reference statements)

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“…The average b value is $0.9 Å À1 , in good agreement with transport data across alkyl chain monolayers in AuÀSÀalkyl/Au junctions. [23] Equation (6a) is a general description of tunneling, regardless of detailed transport mechanism and, indeed, both J C and b are similar for the two analysis methods (Simmons fit (Fig. 5a) and Eq.…”

Section: Extracting Insulator Parameters From Current-density-voltagementioning

confidence: 72%

“…[21,23] In how far can we carry these understandings over to MOMS? Changing molecule-substrate binding from SiÀCÀ to SiÀOÀCÀ [88] and from (Ga(As)ÀO) 2 ÀP(O) to (Ga)AsÀS [140] or the top contact from a physi (ÀCH 3 /Hg) to a chemisorbed (ÀSÀHg) [134] one, showed clear effects on the semiconductorlimited J-V characteristics, which can be rationalized as a dipole effect on the SBH.…”

Section: The Effect Of Contactsmentioning

confidence: 99%

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Molecules on Si: Electronics with Chemistry

et al. 2009

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Basic scientific interest in using a semiconducting electrode in molecule‐based electronics arises from the rich electrostatic landscape presented by semiconductor interfaces. Technological interest rests on the promise that combining existing semiconductor (primarily Si) electronics with (mostly organic) molecules will result in a whole that is larger than the sum of its parts. Such a hybrid approach appears presently particularly relevant for sensors and photovoltaics. Semiconductors, especially Si, present an important experimental test‐bed for assessing electronic transport behavior of molecules, because they allow varying the critical interface energetics without, to a first approximation, altering the interfacial chemistry. To investigate semiconductor‐molecule electronics we need reproducible, high‐yield preparations of samples that allow reliable and reproducible data collection. Only in that way can we explore how the molecule/electrode interfaces affect or even dictate charge transport, which may then provide a basis for models with predictive power.To consider these issues and questions we will, in this Progress Report, review junctions based on direct bonding of molecules to oxide‐free Si. describe the possible charge transport mechanisms across such interfaces and evaluate in how far they can be quantified. investigate to what extent imperfections in the monolayer are important for transport across the monolayer. revisit the concept of energy levels in such hybrid systems.

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Section: Extracting Insulator Parameters From Current-density-voltagementioning

confidence: 72%

Section: The Effect Of Contactsmentioning

confidence: 99%

Molecules on Si: Electronics with Chemistry

et al. 2009

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“…[15][16][17][18][19][20][21][22] The main result of those prior studies is that the current decays exponentially with chain length with a decay constant β ∼ 1.0 per CH 2 unit, with some variation across different experiments. [15][16][17][18][19]23 This dependence has been also successfully addressed computationally using Density Functional Theory (DFT) combined with non-equilibrium Green's function (NEGF) studies. 16,22,24 Yoon et al measured β ∼ 0.9 per CH 2 unit -a value that is in agreement with these earlier results.…”

Section: Introductionmentioning

confidence: 99%

Charge Transport Across Insulating Self-Assembled Monolayers: Non-equilibrium Approaches and Modeling To Relate Current and Molecular Structure

et al. 2014

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This paper examines charge transport by tunneling across a series of electrically (insulating molecules with the structure HS(CH 2 ) 4 CONH(CH 2 ) 2 R) in the form of selfassembled monolayers (SAMs), supported on silver. The molecules examined were studied experimentally by Yoon et al. (Angew. Chem. Int. Ed., 51, 4658−4661, 2012), using junctions of the structure AgS(CH 2 ) 4 CONH(CH 2 ) 2 R//Ga 2 O 3 /EGaIn. The tail group R had approximately the same length for all molecules, but a range of different structures. Changing the R entity over a range of different structures (aliphatic to aromatic) does not influence the conductance significantly. To rationalize this surprising result, we investigate transport through these SAMs theoretically, using both full quantum methods and a generic, independent-electron tight-binding toy model. We find that the frontier orbitals HOMO, HOMO-1, HOMO-2 and HOMO-3, have similar structures for all of the different R-groups, and that the HOMO, which is largely responsible for the transport in these molecules, is always strongly localized on the thiol group. The relative insensitivity of the current density to the structure of the R group is due not only to these similar frontier orbitals but also to a combination of energy levels (ε) and tunneling amplitudes (t), which lead to similar conductance for different tail groups, i.e. the large difference between ε's in conjugated and saturated groups is compensated by tunneling amplitudes inside the tail group. In addition, the coupling of the tail group to the alkane chain is much weaker for conjugated groups than for saturated ones. Conductance change in these molecules is not influenced by the broadening of the molecular levels connected to the left and right electrodes, since in all these molecules the effective coupling to the silver substrate is the same, whereas the other effective couplings are determined by the weak connection to the oxide Ga 2 O 3 layer. All these factors combine to produce currents largely insensitive to the R group.An estimation of the attenuation constant, β, of the simplified Simmons equation obtained using these methods is β = 1.1 per CH 2 group, in good agreement with observed results. Our analysis sketches the potential landscapes across which tunneling occurs, and suggests that it will be difficult to change rates of tunneling by simple changes 2 in molecular structure. This work indicates that significant control over SAMs largely composed of nominally insulating groups may be possible when tail groups are used that are significantly larger than those used in the experiments of Yoon et al. 13

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“…[13][14][15][16][17] In short, even for one of the simplest systems, i.e. that of alkyl chains between Au electrodes, there is a large discrepancy between the average tunnel barrier height, extracted from current -voltage (I − V ) measurements (∼ 1.2 eV), 14,[18][19][20][21] and the barrier that is expected from the experimentally determined electrode work function and alkyl monolayer ionization potential, i.e. the energy difference between the Fermi level and highest occupied molecular orbital (HOMO) as found by Ultraviolet Photoelectron Spectroscopy, UPS (∼ 5 eV).…”

Section: Introductionmentioning

confidence: 99%

“…This finding agrees with complementary spectroscopic measurements of the samples without Hg contact, but challenges the prevailing concept of HOMO-dominated transport. [13][14][15][16]23,24 Highly doped n ++ -and p ++ -Si-C n H 2n+1 /Hg junctions (n = 2, 14,16,18) were prepared by alkylation of freshly etched P-or B-doped (N d/a ≈ 10 19 cm −3 ) Si(1 1 1) surfaces. Detailed descriptions of the preparation and characterization of both 'long' (C14-C18) and 'short' alkyl chain (C2) samples can be found elsewhere 26,27,69 …”

Section: Introductionmentioning

confidence: 99%

Charge transport across metal/molecular (alkyl) monolayer-Si junctions is dominated by the LUMO level

Yaffe

Scheres

et al. 2012

Phys. Rev. B

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We compare the charge transport characteristics of heavy doped p ++ -and n ++ -Si-alkyl chain/Hg junctions. Photoelectron spectroscopy (UPS, IPES and XPS) results for the molecule-Si band alignment at equilibrium show the Fermi level to LUMO energy difference to be much smaller than the corresponding Fermi level to HOMO one. This result supports the conclusion we reach, based on negative differential resistance in an analogous semiconductor-inorganic insulator/metal junction, that for both p ++ -and n ++ -type junctions the energy difference between the Fermi level and LUMO, i.e., electron tunneling, controls charge transport. The Fermi level-LUMO energy difference, experimentally determined by IPES, agrees with the non-resonant tunneling barrier height deduced from the exponential length-attenuation of the current.

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Electrical conduction through single molecules and self-assembled monolayers

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Cited by 363 publications

References 168 publications

Molecules on Si: Electronics with Chemistry

Molecules on Si: Electronics with Chemistry

Charge Transport Across Insulating Self-Assembled Monolayers: Non-equilibrium Approaches and Modeling To Relate Current and Molecular Structure

Charge transport across metal/molecular (alkyl) monolayer-Si junctions is dominated by the LUMO level

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