Open access peer-reviewed chapter

Ferrocenes as One-Electron Donors in Unimolecular Rectifiers

Written By

Robert Melville Metzger

Submitted: June 13th, 2018 Reviewed: March 25th, 2019 Published: September 30th, 2019

DOI: 10.5772/intechopen.86030

Chapter metrics overview

758 Chapter Downloads

View Full Metrics


Ferrocene is a good electron donor, and as such has been used to test asymmetric conduction (rectification) in molecules that contain ferrocene. Of the five ferrocene-containing molecules that rectify (structures 11, 15, 19, 20, and 22), the last (22) exhibits a record rectification ratio, which should be a dramatic incentive for searching for more high-efficiency rectifiers.


  • ferrocene
  • unimolecular electronics
  • rectification ratio
  • highest occupied molecular orbital
  • Aviram-Ratner proposal of 1974

1. Introduction

“Unimolecular electronics” (UME) [1] was born in 1974 with a theoretical proposal by Arieh Aviram and Mark Ratner (AR) for a one-molecule rectifier (or diode) of electrical current donor-bridge-acceptor (D-σ-A) [1] (Figure 1, structures 1 and 2): within that molecule D represents a π-electron-rich one-electron donor (D) moiety, σ is a short and saturated bridge of sp3-hybridized C atoms (between two and maybe eight C atoms long), and A is the electron-poor moiety that can act a one-electron acceptor. One small correction, AR had suggested that the first mechanistic step would move electron and hole from metal electrodes to the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and the second step would involve the relaxation of the excited-state zwitterion [1]. The experimental direction of rectification for D-σ-A molecules has been shown to be “anti-AR” (Figure 1 structure 3): in step (1), under applied electric field, the neutral ground-state molecule D-σ-A forms an excited-state zwitterion D+-σ-A; in step (2) the electron and hole are transferred to the metal electrodes [2].

Figure 1.

The Aviram-Ratner proposal of unimolecular rectification [1] with two specific molecules suggested (1, 2). Structure 3 shows the rectification direction (direction of larger and favored electron flow) seen (i) experimentally (bottom arrow from left to right, see Ref. [16])) and (ii) the rectification direction predicted by Aviram and Ratner (top arrow from right to left crossed out). Structures 418 are the unimolecular rectifiers studied at the University of Alabama (1997–2018) [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]: listed are the direction of enhanced current (hollow arrow), the rectification ratio RR = −I(Vmax)/I(−Vmax), and the maximum bias Vmax(Volts) measured; the word “decays” means that RR decreases monotonically as the measurement is repeated, while “persistent” means that RR does not decrease. The electron donor regions are shown in red, and the electron acceptor regions are shown in blue.

The first rectifier (4 in Figure 1) was measured in 1990–1993 as a Langmuir-Blodgett (LB) multilayer between dissimilar metal electrodes by J. Roy Sambles (University of Exeter) and Geoffrey J. Ashwell (Cranfield University) [3, 4]. The asymmetric electrical current was confirmed at the University of Alabama (UA) as a LB monolayer of 4 between Al electrodes in 1997 [5] and then between oxide-free Au electrodes in 2001 [6, 7].

As of 2015, 53 unimolecular rectifiers had been measured worldwide [8], 15 of which at the UA (Figure 1, structures 418 [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]). Also, 169 molecular wires were measured around the world [8]. Several more rectifiers have been published worldwide since and many review articles on this subject have appeared [21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32].

The present article focuses on the use of one particular powerful one-electron donor in rectifiers: ferrocene.


2. Results

In the 1980s, UME had hoped to develop useful molecular-scale (~2 nm3) devices for ultrahigh-density and high-speed industrial electronics. To interrogate such molecules (or monolayers of molecules), metal electrodes or nanoelectrodes (Al, Ag, Au, etc.) are used: this is sketched below and explained in detail in many review articles [8, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32]. UME learned how inorganic metals couple (associate with or bond to) single organic molecules and how one can reliably exchange electrons and photons with these molecules.

In the meantime the much wealthier and commercially driven electronic industry has made gigantic strides and has developed high-speed almost-nanoscale electronic circuits using inorganic semiconductors (Si, Ge, and GaAs). Therefore the original hope and promise of UME have been defeated. Nevertheless, UME has learned to interrogate and control individual molecules, and its present challenge is how to combine and exploit electronic, photonic, and spintronic functions in new ways.

The present review looks at how ferrocene-containing molecules have contributed valid and promising and most recently also very dramatic UME rectifiers: in particular molecules 11 [14] and 15 [17] already mentioned in Figure 1 and four other molecules shown in Figure 2, 19 [33, 34], 20 [33, 34], and 21 [33, 34] studied by the Whitesides group at Harvard University and 22 [36] studied by the Nijhuis group at Singapore National University. As discussed below, molecule 21 should not, and does not, rectify.

Figure 2.

Ferrocene-containing unimolecular rectifiers studied by the groups of Whitesides [33, 34, 35] and Nijhuis [36]: all are “asymmetry-type A” rectifiers; the hollow arrow denotes the preferred direction of electron flow through the “metal | molecule | metal” sandwich (from electrode far from the electron donor part to the nearest electrode). Corrigenda for Ref. [8]: (i) the arrows for 19 (i.e., “molecule 193”) and 20 (i.e., “molecule 196”) are drawn in the wrong direction: they would apply to D-σ-A rectifiers); (ii) for 20 (i.e., “molecule 196”), the reference in Ref. [8] should be [124] instead of [122], [123]; and (iii) for 21 (i.e., “molecule 197”), the reference in Ref. [8] should be [125] instead of [122], [123].

Electrical measurements of rectifiers. Rectification can be measured with some difficulty at the single-molecule level, but more conveniently as a monolayer between macroscopic metal electrodes as a “metal | molecule | metal” sandwich [32]. All molecules discussed here were studied either as a Langmuir-Blodgett monolayer (11 [14]) or as self-assembled monolayers (SAMs) with thiol terminations that could be bound covalently to either super-flat “template-stripped” AgTS or ATS electrodes (15 [17], 19 [33, 34], and 20 [34]) or PtTS electrodes (22 [37]). Most (but not all) rectifier measurements have been done with direct current [8, 32].

Candidate unimolecular rectifiers can be (i) electron donor molecules, (ii) electron acceptor molecules, or (iii) D-σ-A molecules [8, 32]. The search for organic rectifiers started in the era of quasi-one-dimensional organic metals with an enduring synthetic emphasis toward strong (easily oxidized) donor moieties D and strong (easily reduced) acceptor moieties A, connected by a covalently saturated and electrically insulating “sigma” (σ) bridge, forming a D-σ-A molecule. For instance, the proposed tetrathiafulvalene (TTF)-σ-tetracyanoquinodimethane molecule (Figure 1, structure 1) would have a presumed low barrier to form the corresponding excited zwitterion D+-σ-A. Yet in 1974, a (weak donor)-σ-(weak acceptor) molecule (Figure 1, structure 2) was also proposed [1]. Surprisingly, the recently studied (and dimensionally very tiny) molecule 18 (that resembles molecule 2) incorporates a weak electron donor D and a moderate electron acceptor A, yet is an excellent rectifier [20]. We have also seen that a strong, easily oxidized donor like tetramethyl-para-phenylenediamine in molecule 16 blocks the current across the monolayer between −0.5 and +0.5 Volts (Coulomb blockade) [18].

Table 1 shows some relevant gas-phase ionization potentials ID for electron donors D and gas-phase electron affinities AA for electron acceptors A [8]. It should be noted that ferrocene (Fc or Cp2Fe, where Cp is cyclopentadienyl) is as good an electron donor (i.e., has a relatively small ID value) as tetrathiafulvalene TTF, but not as good as N,N,N′,N′-tetramethyl-para-phenylenediamine (TMPD). Perylenebisimide (PBI) is as good an electron acceptor (i.e., has a similarly large AA value) as 7,7,8,8-tetracyanoquinodimethane (TCNQ).

Fc = Cp2Fe6.81b

Table 1.

Gas-phase ionization potentials ID (eV) and gas-phase electron affinities AA(eV), updated from [8], except where noted.

From Ref. [42].

From Ref. [43].

Calculated from Ref. [18].

Four mechanisms for rectification. Four potential mechanisms for electrical rectification in molecules have been discussed [38, 39, 40]:

  1. Schottky barriers (“S” rectifier) [38, 40].

  2. Asymmetric placement of the electrophore in the electrode gap (“A” rectifier) [38].

  3. Unimolecular processes depending on molecular energy levels (“U” rectifier) [38].

  4. A recent fourth mechanism for rectification is asymmetric polarization (“AP” rectifier), when highly polar solvents can induce an asymmetric conductance of a symmetrical molecule between very asymmetric electrodes in a scanning break junction (SBJ) [41].

Figure 3.

IV scans for a “Au | LB → SAM of 15 | Cold Au | Ga2O3 | EGaIn” sandwich in the bias range from −1.0 to +1.0 Volts for the bias Vincreasing: (left) I (ampères) vs. V (Volts). (Center) log10I vs. V. (Right) RR(V)°−I(V)/I(−V) vs. V: (the average < RR > = 96.3 ± 36.7). The horizontal arrows indicate the scan direction; the vertical hollow arrows show how the ordinate values evolved with repeated scans [18].

Figure 4.

Current-voltage (IV) curves (I/ampères vs. V/Volts) for a “EGaIn | Ga2O3 | Au | Z-type LB monolayer of 11 | Cold Au | Ga2O3 | EGaIn” sandwich. (left) I vs. V. (Center) log10|I| vs. V. (Right) RR = 14–28 vs. V. RR persists for up to 40 measurement cycles, with a minimal decrease in the currents (which are relatively small). Rectification was even seen for biases up to ±2 Volts [14].

Purists would prefer pure-“U” rectifiers, requiring “S” = 0 and “A” = 0. For many molecules, for reasons of assembly, “U” and “A” effects are combined [39] (e.g., Figure 1 for structures 4, 5, 6, 10, 11, 14, 15, 16). For molecules 19, 20, and 22 in Figure 2, only the “A” effect is operative: the chromophore donor moiety (indicated as “D” and shown in red) yields rectifiers because it is asymmetrically placed within the “metal | monolayer | metal” sandwich. When the D moiety is in the middle of molecule 21, there can be and is no rectification [33, 34], as predicted [38].

Reversal of rectification: “Janus effect.” The molecules studied routinely at the UA rectify in the “anti-AR” direction, that is, intramolecular electron flow occurs from D to A (e.g., Figure 1 structure 3) [2, 8]. However, D-σ-A rectifiers 13, 14, 15, and 16 also show an additional “Janus effect”: at lower bias they rectify one way, and at higher bias (e.g., at ±2.5 V), they rectify the other way [17, 18]! At lower bias, AR rectification may involve only one energy level (e.g., LUMO); at higher bias, anti-AR rectification may involve both HOMO and LUMO. Such bias-switchable rectifiers may be useful!

Rectification ratio. The asymmetry in electrical current I is quantified by the rectification ratio:


where V is the applied bias or voltage. Typical RR values span several orders of magnitude; for the rectifiers 418 studied at the UA, RR(Vmax) is reported in Figure 1. The first rectifier, 4, had RR = 26 [7]; 16 has a large RR = 3000 [19]. Why is the RR typically seen for unimolecular rectifiers (RR ≤ 103) [8] so much smaller than the RR for commercial inorganic pn junction devices (RR = 105–106) [8]? If low RRs were intrinsic to UME rectifiers, then traditional Ge, Si, and Ga As semiconductor physicists could safely look down at UME as a harmless curiosity, not as a competitor. But, as discussed next, a huge increase in RRs was imminent.

We next discuss rectifier 16, in which the electron donor moiety is the powerful electron donor TMPD instead of ferrocene (Figure 5). Figure 6 shows a surprisingly large room temperature Coulomb blockade [18]: too much of a good thing, the TMPD oxidizes too easily and prevents current from flowing for a large bias range [18]! Beyond where the Coulomb blockade was operative, a relatively impressive RR ≈ 3000 is reached.

Figure 5.

IV scans for a “EGaIn | Au | LB → SAM of 15 | Cold Au | EGaIn” sandwich in the bias range from −2.5 to +2.5 Volts for the bias Vincreasing: (left) I (ampères) vs. V (Volts). (Center) log10I vs. V. Note that the position of minimum current, which for normal tunneling curves of this type should occur at zero volt bias, as in Figures 3(center) and 4 (center) shown above, is displaced here very significantly to the left by about 0.8 Volts: This is incipient Coulomb blockade. (Right) RR(V)°–I(V)/I(−V) vs. V [18].

Figure 6.

IV scans for a “EGaIn | Au | LB → SAM of 16 | Cold Au | EGaIn” sandwich in the bias range from −2.5 to +2.5 Volts for the bias Vincreasing: (left) I (ampères) vs. V (Volts) (average of 50 scans). (Center) log10I vs. V. True Coulomb blockade. (C) RR(V)°−I(V)/I(−V) vs. V. (right) RR vs. V [18].

The Whitesides group (including Nijhuis) studied the rectification of self-assembled monolayers of thiol-containing molecules 19 and 20 and the non-rectification of the symmetric 22 [35], in sandwich “Au | SAM | Ga2O3 | GaIn” with a thorough effort to isolate the potential influence of the disordered Ga2O3 oxide that forms at the surface of the GaIn eutectic (without completely covering it) [33, 34, 35] (Figure 7).

Figure 7.

JV scans for “AgTS | SAM of alkanethiol CH3(CH2)11SH | Ga2O3 | EGaIn” sandwich in the bias range from −1.0 to +1.0 Volts, (A) J vs. V (B) detail of (A) showing hysteresis (C) log10 J vs. V. RR ≈ 100. From Ref. [33, 34].

Recent huge rectification ratio. A very dramatic result was published recently for the (“A-type”) rectifying monolayer sandwich “PtTS-S-C15H30-Fc-C≡C-Fc | EGaIn” (Figure 2, structure 22) consisting of a diad of ferrocene (Fc) donors (linked by an alkynyl-C ≡ C-), with a pentadecanethiol “tail” [36]. This sandwich was studied between a bottom template-stripped electrode MTS (=PtTS, AuTS, or AgTS) and an EGaIn droplet top electrode. The new record is a very dramatic RR = 6.3 × 105 at ±3 Volts for PtTS (but much less for AuTS or AgTS) (Figure 8) [36]. Also, the conductance “plateaued” around −2 Volts when the AgTS electrode was used [36].

Figure 8.

IV data for “PtTS | SAM of 22 | Ga2O3 | EGaIn” sandwich: (A) log10J vs. V and (B) rectification ratio RR(V) vs. V. the current densities J = I/A are calculated from the measured currents I and the estimated areas A of the EGaIn drops. The “heat map” shows in false color the number of times that any point in the xy plot was recorded (see color code on the right of each xy plot) From Ref. [36].

The key improvements in [36] were (i) using Pt as the “bottom” electrode, because PtTS tolerates a larger bias range than AuTS or AgTS, (ii) a presumed efficient van der Waals contact between Fc-C ≡ C-Fc and EGaIn, and (iii) a “long enough alkyl tail” to get a very small reverse-bias current [38].

Also, light emission was measured (with blinking) for 22, with a broad peak at 1.7 eV (λmax = 730 nm), but only at the large negative bias V that corresponds to rectification: this emission was attributed to surface plasma polaritons excited distally within the Pt electrode after tunneling. Thus, the electrical excitation at large negative bias may have accessed the HOMO and HOMO-1 of Fc, but the energy is emitted neither directly (electroluminescence from Fc+ with an expected narrow energy distribution) nor indirectly (as lattice phonons), but indirectly and effectively, as surface plasma polaritons with a wide spectral distribution [36].


3. Conclusion

The frustrating issue of historically low measured RRs [8] has thus been resolved experimentally [36]: organic monolayer rectifiers may finally challenge the RR of inorganic pn junction rectifiers.

However, the measured RRs for alkanethiols are hundreds of times smaller than expected from careful theoretical simulations [42]: this puzzle must be solved, so that measurements are not victims of unforeseen inefficiencies in the “metal | molecule” interface. The number of measured unimolecular rectifiers has grown dramatically, but their preselection as candidate rectifiers has been somewhat haphazard. Once the “metal | molecule” interface is brought under experimental control, better measurements may provide valid physical organic criteria to guide the design of the better unimolecular rectifiers of tomorrow.

There has also been a recent brief review on this exact topic [43]; for the sake of brevity, we refer the reader to the papers cited for other significant rectifiers containing the donor ferrocene [43, 44, 45, 46, 47, 48, 49].


  1. 1. Aviram A, Ratner MA. Molecular rectifiers. Chemical Physics Letters. 1974;29:277-283
  2. 2. Metzger RM, Mattern DL. Unimolecular electronic devices. In: Metzger RM, editor. Unimolecular and Supramolecular Electronics II: Chemistry and Physics Meet at the Metal-Molecule Interface. Vol. 313. Heidelberg, Dordrecht, London, New York: Springer Topics in Current Chemistry; 2012. pp. 39-84
  3. 3. Ashwell GJ, Sambles JR, Martin AS, Parker WG, Szablewski M. Rectifying characteristics of Mg | (C16H33-Q3CNQ LB film) | Pt structures. Journal of the Chemical Society, Chemical Communications. 1990:1374-1376
  4. 4. Martin AS, Sambles JR, Ashwell GJ. Molecular rectifier. Physical Review Letters. 1993;70:218-221
  5. 5. Metzger RM, Chen B, Höpfner U, Lakshmikantham MV, Vuillaume D, Kawai T, et al. Unimolecular electrical rectification in hexadecyl-quinolinium tricyanoquinodimethanide. Journal of the American Chemical Society. 1997;119:10455-10466
  6. 6. Xu T, Peterson IR, Lakshmikantham MV, Metzger RM. Rectification by a monolayer of hexadecylquinolinium tricyanoquinodimethanide between gold electrodes. Angewandte Chemie International Edition. 2001;40:1749-1752
  7. 7. Metzger RM, Xu T, Peterson IR. Electrical rectification by a monolayer of hexadecylquinolinium tricyanoquinodimethanide measured between macroscopic gold electrodes. The Journal of Physical Chemistry. B. 2001;105:7280-7290
  8. 8. Metzger RM. Unimolecular electronics. Chemical Reviews. 2015;115:5056-5115
  9. 9. Honciuc A, Otsuka A, Wang Y-H, McElwee SK, Woski SA, Saito G, et al. Polarization of charge-transfer bands and rectification in hexadecylquinolinium 7,7,8-tricyanoquinodimethanide and its tetrafluoro analog. The Journal of Physical Chemistry. B. 2006;110:15085-15093
  10. 10. Jaiswal A, Rajagopal D, Lakshmikantham MV, Cava MP, Metzger RM. Unimolecular rectification and other properties of CH3C(O)S-C14H28Q+-3CNQ and CH3C(O)S-C16H32Q+-3CNQ organized by self-assembly, Langmuir-Blodgett, and Langmuir-Schaefer techniques. Physical Chemistry Chemical Physics. 2007;9:4007-4017
  11. 11. Baldwin JW, Amaresh RR, Peterson IR, Shumate WJ, Cava MP, Amiri MA, et al. Rectification and nonlinear optical properties of a Langmuir-Blodgett monolayer of a pyridinium dye. The Journal of Physical Chemistry. B. 2002;106:12158-12164
  12. 12. Metzger RM, Baldwin JW, Shumate WJ, Peterson IR, Mani P, Mankey GJ, et al. Large current asymmetries and potential device properties of a Langmuir-Blodgett monolayer of dimethyanilinoazafullerene sandwiched between gold electrodes. The Journal of Physical Chemistry. B. 2003;107:1021-1027
  13. 13. Honciuc A, Jaiswal A, Gong A, Ashworth K, Spangler CW, Peterson IR, et al. Current rectification in a Langmuir-Schaefer monolayer of fullerene-bis-[4-diphenylamino-4"-(N-ethyl-N-2"-ethyl)amino-1,4-diphenyl-1,3-butadiene] malonate between Au electrodes. The Journal of Physical Chemistry. B. 2005;109:857-871
  14. 14. Shumate WJ, Mattern DL, Jaiswal A, Burgess J, Dixon DA, White TR, et al. Spectroscopic and rectification studies of three donor sigma-acceptor compounds, consisting of a one-electron donor (pyrene or ferrocene), a one-electron acceptor (perylenebisimide), and a C19 swallowtail. The Journal of Physical Chemistry. B. 2006;2006(110):11146-11159
  15. 15. Shumate WJ. Ph. D. Dissertation, University of Alabama; 2007
  16. 16. Honciuc A, Metzger RM, Gong A, Spangler CW. Elastic and inelastic electron tunneling spectroscopy of a new rectifying monolayer. Journal of the American Chemical Society. 2007;129:8310-8319
  17. 17. Johnson MS, Kota R, Mattern DL, Hill CM, Vasiliu M, Dixon DA, et al. A two-faced “Janus” unimolecular rectifier exhibits rectification reversal. Journal of Materials Chemistry C. 2014;2:9892-9902
  18. 18. Johnson MS, Kota R, Mattern DL, Metzger RM. Janus reversal and coulomb blockade in ferrocene-perylenebisimide and N,N,N,'N'-tetramethyl-para-phenylene-diamine-perylenebisimide D-σ-A rectifiers. Langmuir. 2016;32:6851-6859
  19. 19. Johnson MS, Wickramasinghe L, Verani C, Metzger RM. Confirmation of the rectifying behavior in a pentacoordinate [N2O3] iron(III) surfactant using a eutectic Ga-In | LB monolayer | Au assembly. Journal of Physical Chemistry C. 2016;2016(120):10578-10583
  20. 20. Meany JE, Johnson MS, Woski SA, Metzger RM. Surprisingly big rectification ratios for a very small unimolecular rectifier. ChemPlusChem. 2016;81:1152-1155
  21. 21. Tour JM, Kozaki M, Seminario JM. Molecular-scale electronics: A synthetic/computational approach to digital computing. Journal of the American Chemical Society. 1998;120:8486-8493
  22. 22. Love JC, Estroff LA, Kriebel JK, Nuzzo RG, Whitesides GM. Self-assembled monolayers of thiolates on metals as a form of nanotechnology. Chemical Reviews. 2005;105:1103-1169
  23. 23. Aradhya SV, Venkataraman L. Single-molecule junctions beyond electronic transport. Nature Nanotechnology. 2013;8:399-409
  24. 24. Sun L, Diaz-Fernandez YA, Gschneidtner TA, Westerlund F, Lara-Avila S, Moth-Poulsen K. Single-molecule electronics: From chemical design to functional devices. Chemical Society Reviews. 2014;43:7378-7411
  25. 25. Nichols RJ, Higgins SJ. Single-molecule electronics: Chemical and analytical perspectives. Annual Review of Analytical Chemistry. 2015;8:389-417
  26. 26. Lambert CJ. Basic concepts of quantum interference and electron transport in single-molecule electronics. Chemical Society Reviews. 2015;44:875-888
  27. 27. Xiang D, Wang X, Jia C, Lee T, Guo X. Molecular-scale electronics: From concept to function. Chemical Reviews. 2016;116:4318-4440
  28. 28. Su TA, Neupane M, Steigerwald ML, Venkataraman L, Nuckolls C. Chemical principles of single-molecule electronics. Nature Reviews Materials. 2016;1:16002
  29. 29. Vilan A, Aswal D, Cahen D. Large-area, ensemble molecular electronics: Motivation and challenges. Chemical Reviews. 2017;117:4248-4286
  30. 30. Vilan A, Cahen D. Chemical modification of semiconductor surfaces for molecular electronics. Chemical Reviews. 2017;117:4624-4666
  31. 31. Cuevas JC, Scheer E. Molecular Electronics. Second ed. Singapore: World Scientific; 2017
  32. 32. Metzger RM. Quo Vadis unimolecular electronics? Nanoscale. 2018;10:10316-10332
  33. 33. Nijhuis CA, Reus WF, Whitesides GM. Molecular rectification in metal-SAM-metal oxide-metal junctions. Journal of the American Chemical Society. 2009;131:17814-17827
  34. 34. Nijhuis C, Reus WF, Barber JR, Dickey MD, Whitesides GM. Charge transport and rectification in arrays of SAM-based tunneling junctions. Nano Letters. 2010;10:3611-3619
  35. 35. Nijhuis C, Reus WF, Siegel AC, George M, Whitesides. A molecular half-wave rectifier. Journal of the American Chemical Society. 2011;133:15397-154111
  36. 36. Reus WF, Thuo MM, Shapiro ND, Nijhuis CA, Whitesides GM. The SAM, not the electrodes, dominates charge transport in metal-monolayer//Ga2O3/gallium-indium eutectic junctions. ACS Nano. 2012;6:4806-4822
  37. 37. Chen X, Roemer M, Yuan L, Du W, Thompson D, del Barco E, et al. Molecular diodes with rectification ratios exceeding 105 driven by electrostatic interactions. Nature Nanotechnology. 2017;12:797-803
  38. 38. Krzeminski C, Delerue C, Allan G, Vuillaume D, Metzger RM. Theory of rectification in a molecular monolayer. Physics Review. 2001;B64:085405
  39. 39. Mujica V, Ratner MA, Nitzan A. Molecular rectification: Why is it so rare? Chemical Physics. 2002;281:147-150
  40. 40. Van Dyck C, Ratner M. Molecular rectifiers: A new design based on asymmetric anchoring moieties. Nano Letters. 2015;15:1577-1584
  41. 41. Capozzi B, Xia J, Adak O, Dell EJ, Liu Z-F, Taylor JC, et al. Single-molecule diodes with high rectification ratios through environmental control. Nature Nanotechnology. 2015;10:522-527
  42. 42. Xie Z, Baldea I, Frisbie CD. Why one can expect large rectification in molecular junctions based on alkane monothiols and why rectification is so modest. Chemical Science. 2018;9:4456-4467
  43. 43. Welker ME. Ferrocenes as building blocks in molecular rectifiers and diodes. Molecules. 2018;23:1551
  44. 44. Wimbush KS, Reus WF, van der Wiel WG, Reinhoudt DN, Whitesides GM, Nijhuis CA, et al. Control over rectification in supramolecular tunneling junctions. Angewandte Chemie, International Edition. 2010;49:10176-10180
  45. 45. Song J, Vancso GJ. Responsive organometallic polymer grafts: Electrochemical switching of surface properties and current mediation behavior. Langmuir. 2011;27:6822-6829
  46. 46. Mentovich ED, Rosenberg-Shraga N, Kalifa I, Gozin M, Mujica V, Hansen T, et al. Gated-controlled rectification of a self-assembled monolayer-based transistor. Journal of Physical Chemistry C. 2011;117:8468-8474
  47. 47. Park S, Park JH, Hwang S, Kwak J. Programmable electrochemical rectifier based on a thin-layer cell. ACS Applied Materials & Interfaces. 2017;9:20955-20962
  48. 48. Broadnax AD, Lamport ZA, Scharmann B, Jurchescu OD, Welker ME. Ferrocenealkylsilane molecular rectifiers. Journal of Organometallic Chemistry. 2018;856:23-26
  49. 49. Zhang G-P, Mu YQ , Wei MZ, Wang S, Huang H, Hu GC, et al. Designing molecular rectifiers and spin valves using metallocene-functionalized undecanethiolates: One transition metal Atom matters. Journal of Materials Chemistry C. 2018;6:2105-2112

Written By

Robert Melville Metzger

Submitted: June 13th, 2018 Reviewed: March 25th, 2019 Published: September 30th, 2019