Transition from Direct Tunneling to Field Emission in Metal-Molecule-Metal Junctions

Beebe, Jeremy M.; Kim, BongSoo; Gadzuk, J. W.; Frisbie, C. Daniel; Kushmerick, James G.

doi:10.1103/physrevlett.97.026801

Cited by 572 publications

(784 citation statements)

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“…[34,161,162] Experimentally, it is fairly simple to identify this transition bias (V T ) and indeed, in the study cited, [34,161,162] V T correlated well with the expected barrier height, the difference of the molecular level (highest occupied molecular orbital, HOMO) and the electrode Fermi level as deduced independently by UPS (see, however, Section 5.2). [52,161,162] This approach assumes a roughly linear J-V relation for tunneling transport [161] (i.e., positive ln(J/V 2 ) vs. 1/V slope), and in that case V T indeed indicates a change in transport mechanism to field-emission. However, as the barrier width/height ratio (which we called ''shape factor'' [149,159] ) increases, the tunneling J-V becomes more non-linear.…”

Section: Extracting Insulator Parameters From Current-density-voltagementioning

confidence: 52%

“…[149,159,161,162] One approach postulates that if the applied bias is larger than the barrier height, the transport mechanism changes from direct tunneling to field-emission. [34,161,162] Experimentally, it is fairly simple to identify this transition bias (V T ) and indeed, in the study cited, [34,161,162] V T correlated well with the expected barrier height, the difference of the molecular level (highest occupied molecular orbital, HOMO) and the electrode Fermi level as deduced independently by UPS (see, however, Section 5.2). [52,161,162] This approach assumes a roughly linear J-V relation for tunneling transport [161] (i.e., positive ln(J/V 2 ) vs. 1/V slope), and in that case V T indeed indicates a change in transport mechanism to field-emission.…”

Section: Extracting Insulator Parameters From Current-density-voltagementioning

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: 52%

Section: Extracting Insulator Parameters From Current-density-voltagementioning

confidence: 99%

Molecules on Si: Electronics with Chemistry

et al. 2009

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“…The difference in β for aliphatic and aromatic carboxylates is in agreement with predictions based on molecular orbital (MO) theory. 2,4,8 Aromatic molecules are characterized by smaller energy gaps (∼3−5 eV) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) than those of aliphatic molecules (∼7 eV) of similar length. 2,48 Furthermore, the HOMO of aromatic molecules aligns more favorably with the Fermi level of electrodes than does that of aliphatic molecules; this alignment, in a SAM-based tunneling junction, facilitates charge transport by lowering the effective height of the tunneling barrier.…”

Section: ■ Backgroundmentioning

confidence: 99%

“…Using a potential barrier model that explicitly assumes constant contributions to rates of tunneling from the interfaces between the SAMs and the electrodes, we found that (CH 2 ) n and (C 6 H 4 ) m segments contribute independently but differently to the shape of the tunneling barrier and that the values of β for (CH 2 ) n and (C 6 H 4 ) m are independent of the order in which they are assembled. 4 We expect this conclusion to hold only when the molecular units (R) being considered are isolated electronically from strong interactions with the electrodes.…”

Section: ■ Conclusionmentioning

confidence: 99%

Molecular Series-Tunneling Junctions

et al. 2015

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Charge transport through junctions consisting of insulating molecular units is a quantum phenomenon that cannot be described adequately by classical circuit laws. This paper explores tunneling current densities in self-assembled monolayer (SAM)-based junctions with the structure Ag TS / O 2 C−R 1 −R 2 −H//Ga 2 O 3 /EGaIn, where Ag TS is templatestripped silver and EGaIn is the eutectic alloy of gallium and indium; R 1 and R 2 refer to two classes of insulating molecular units(CH 2 ) n and (C 6 H 4 ) m that are connected in series and have different tunneling decay constants in the Simmons equation. These junctions can be analyzed as a form of series-tunneling junctions based on the observation that permuting the order of R 1 and R 2 in the junction does not alter the overall rate of charge transport. By using the Ag/O 2 C interface, this system decouples the highest occupied molecular orbital (HOMO, which is localized on the carboxylate group) from strong interactions with the R 1 and R 2 units. The differences in rates of tunneling are thus determined by the electronic structure of the groups R 1 and R 2 ; these differences are not influenced by the order of R 1 and R 2 in the SAM. In an electrical potential model that rationalizes this observation, R 1 and R 2 contribute independently to the height of the barrier. This model explicitly assumes that contributions to rates of tunneling from the Ag TS /O 2 C and H//Ga 2 O 3 interfaces are constant across the series examined. The current density of these series-tunneling junctions can be described by J(V) = J 0 (V) exp(−β 1 d 1 − β 2 d 2 ), where J(V) is the current density (A/cm 2 ) at applied voltage V and β i and d i are the parameters describing the attenuation of the tunneling current through a rectangular tunneling barrier, with width d and a height related to the attenuation factor β.

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“…It can be compared to a tunneling barrier, which charge carriers have to overcome to generate a current. Ultraviolet photoelectron spectroscopy (UPS) 1 , thermopower [2][3][4] , and transition voltage spectroscopy (TVS) [5][6][7][8][9][10] represent current methods to estimate the relative alignment of the dominant molecular orbital from experimental data.…”

Section: Introductionmentioning

confidence: 99%

A quantum chemical study from a molecular transport perspective: ionization and electron attachment energies for species often used to fabricate single-molecule junctions

Bâldea

2014

Faraday Discuss.

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Abstract:The accurate determination of the lowest electron attachment (EA) and ionization (IP) energies for molecules embedded in molecular junctions is important for correctly estimating, e.g., the magnitude of the currents (I) or the biases (V ) where an I −V -curve exhibits a significant non-Ohmic behavior. Benchmark calculations for the lowest electron attachment and ionization energies of several typical molecules utilized to fabricate single-molecule junctions characterized by n-type conduction (4,4'-bipyridine, 1,4-dicyanobenzene, and 4,4'-dicyano-1,1'-biphenyl) and p-type conduction (benzenedithiol, biphenyldithiol, hexanemonothiol, and hexanedithiol] based on the EOM-CCSD (equation-of-motion coupled-cluster singles and doubles) state-of-the-art method of quantum chemistry are presented. They indicate significant differences from the results obtained within current approaches to molecular transport. The present study emphasizes that, in addition to a reliable quantum chemical method, basis sets much better than the ubiquitous double-zeta set employed for transport calculations are needed. The latter is a particularly critical issue for correctly determining EA's, which is impossible without including sufficient diffuse basis functions. The spatial distribution of the dominant molecular orbitals (MO's) is another important issue, on which the present study draws attention, because it sensitively affects the MO-energy shifts Φ due to image charges formed in electrodes. The present results cannot substantiate the common assumption of a point-like MO midway between electrodes, which substantially affects the actual Φ-values.

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Transition from Direct Tunneling to Field Emission in Metal-Molecule-Metal Junctions

Cited by 572 publications

References 27 publications

Molecules on Si: Electronics with Chemistry

Molecules on Si: Electronics with Chemistry

Molecular Series-Tunneling Junctions

A quantum chemical study from a molecular transport perspective: ionization and electron attachment energies for species often used to fabricate single-molecule junctions

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