- Nano Express
- Open Access
Low Tunneling Decay of Iodine-Terminated Alkane Single-Molecule Junctions
Nanoscale Research Letters volume 13, Article number: 121 (2018)
One key issue for the development of molecular electronic devices is to understand the electron transport of single-molecule junctions. In this work, we explore the electron transport of iodine-terminated alkane single molecular junctions using the scanning tunneling microscope-based break junction approach. The result shows that the conductance decreases exponentially with the increase of molecular length with a decay constant βN = 0.5 per –CH2 (or 4 nm−1). Importantly, the tunneling decay of those molecular junctions is much lower than that of alkane molecules with thiol, amine, and carboxylic acid as the anchoring groups and even comparable to that of the conjugated oligophenyl molecules. The low tunneling decay is attributed to the small barrier height between iodine-terminated alkane molecule and Au, which is well supported by DFT calculations. The work suggests that the tunneling decay can be effectively tuned by the anchoring group, which may guide the manufacturing of molecular wires.
Understanding the electron transport of single-molecule junctions is crucial for the development of molecular electronic devices [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. The non-resonant tunneling model has often been used to describe the electron transport process through small molecule, where contact conductance, molecular length, and the tunneling decay constant are the main parameters [17, 18]. In most molecular systems, decay constant is highly related to the electronic properties of organic backbone. For example, the conjugated molecular systems have low tunneling decay, unlike non-conjugated ones [17, 19]. Since the tunneling decay is decided by the barrier height between the Fermi level of electrode and lowest unoccupied molecular orbital (LUMO) or highest occupied molecular orbital (HOMO) of molecular junctions [17, 20], it is possible to tune the molecular energy level towards the Fermi level to achieve the low decay [21,22,23,24].
In single-molecule junctions, the anchoring group plays an important role in the control of electronic coupling between the organic backbones with the electrodes [21, 23,24,25]. A series of conductance measurements for the alkane-based molecules have showed a significant effect of different anchoring groups on the binding geometry, junction formation probabilities, contact conductance, and even conductance channel (through LUMO or HOMO) of molecular junctions [21,22,23,24,25]. Since the anchoring group can regulate the frontier orbitals in the molecular junction, the tunneling decay of the molecule may also be tuned by the anchoring group . However, limited study has been focused on this area.
Herein, we report the electron transport of alkane molecules terminated with iodine group by using scanning tunneling microscopy break junction (STM-BJ) (Fig. 1) [26, 27]. The single molecular conductance measurements show that the conductance decreases exponentially with the increase of molecular lengths and the decay constant of alkane molecules with iodine group is much lower than that of the analogues with other anchoring groups. The different tunneling decay constants for alkane molecules with varied anchoring groups are explained by barrier height between molecule and electrode.
1,4-Butanediiodo, 1,5-pentanediiodo, and 1,6-hexanediiodo were purchased from Alfa Aesar. All solutions were prepared with ethanol. Au(111) was used as the substrate, while mechanically cut Au tips were used as the tips. Before each experiment, the Au(111) was electrochemically polished and carefully annealed in a butane flame and then dried with nitrogen.
The Au(111) substrate was immersed into a freshly prepared ethanol solution containing 0.1 mM target molecules for 10 min. The conductance measurement was carried out on the modified Nanoscope IIIa STM (Veeco, USA.) by using the STM-BJ method at room temperature [28,29,30], which simply measured the conductance of single-molecule junctions formed by repeatedly moving the tip into and out of the substrate at a constant speed. During the process, the molecules could anchor between the two metal electrodes and form single molecular junctions. Thousands of such curves were collected for statistical analysis. All the experiments were performed with a fix bias voltage of 100 mV. Since molecules with iodine as the anchoring group are a photosensitive material, the experiment was carried out under shading.
Results and Discussion
Conductance Measurement of Iodine-Terminated Alkane Single Molecular Junctions
The conductance measurements were first carried out on Au(111) with monolayer of 1,4-butanediiodo by STM-BJ. Figure 2a gives out the typical conductance traces exhibiting the stepwise feature. Conductance traces show plateau at 1 G0, indicating the formation of stable Au atomic contact. Plateau at a conductance value of 10−3.6 G0 (19.47 ns) is also found besides the 1 G0, owing to the formation of molecular junction. A conductance histogram could also be obtained by treating with logarithm and binning of conductance value from more than 3000 conductance traces, and then, the intensity of conductance histogram was normalized by the number of traces used and shows a conductance peak at 10−3.6 G0 (19.44 ns) (Fig. 2b). Those show that the iodine group can serve as an effective anchoring group forming molecular junction. However, this value is smaller than the single molecular conductance value of 1,4-butanediamine with amine as the anchoring group, which may stem from weak interaction between iodine and Au electrode .
In comparison with 1,4-diiodobutane, pronounced peaks at 10−3.8 G0 (12.28 ns) and 10−4.0 G0 (7.75 ns) are found for 1,5-pentanediiodo and 1,6-hexanediiodo, respectively (Fig. 3). The conductance values decrease with the increasing of molecule length. Meanwhile, the conductance values of 1,5-pentanediiodo and 1,6-hexanediiodo are smaller than those of 1,5-pentanediamine and 1,6-hexanediamine, respectively , which may be caused by the different interaction in alkane-based molecular junctions between iodine and amine anchoring groups binding to Au electrodes .
The two-dimensional conductance histograms were also constructed for those molecular junctions (Additional file 1: Figure S1) and give out similar conductance values of one-dimensional histograms. Typically, the breaking off distance of molecular junctions increases with the increasing of molecular length. We also analyze the distance from the conductance value of 10−5.0 G0 to 10−0.3 G0 as shown in Fig. 4, and rupture distances of 0.1, 0.2, and 0.3 nm are found for 1,4-butanediiodo, 1,5-pentanediiodo, and 1,6-hexanediiodo, respectively. Here, the rupture distances are obtained from the maximum peak of the rupture distance histogram . It was reported that there is a snap back distance of 0.5 nm for Au after the breaking of Au–Au contact [34, 35]; thus, the absolute distances for those molecular junctions between electrodes could be 0.6, 0.7, and 0.8 nm which are found for 1,4-butanediiodo, 1,5-pentanediiodo, and 1,6-hexanediiodo, respectively. Those distances are comparable to the length of molecules. Eder et al. reported that the adsorption of 1,3,5-tri (4-iodophenyl)-benzene monolayer onto Au(111) may cause partial dehalogenation ; however, a very larger conductance value for those Au–C covalent contact molecular junctions can be found for molecules with four (around 10−1 G0) and six (bigger than 10−2 G0) –CH2– units . Thus, we propose that the current investigated molecules contact to the Au through the Au–I contact.
Tunneling Decay Constant of Iodine-Terminated Alkane Single Molecular Junctions
Under the current bias, those molecule conductance can be expressed as G = Gc exp(–βNN). Here, G is the conductance of the molecule and Gc is the contact conductance and is determined by the interaction between the anchoring group and the electrode. N is the methylene number in the molecule, and βN is the tunneling decay constant, which reflects the coupling efficiency of electron transport between the molecule and the electrode. As show in Fig. 5, we plot a natural logarithm scale of conductance against the number of methylene; tunneling decay constant βN of 0.5 per –CH2 is determined from the slope of linear fitting. This tunneling decay is very low in alkane-based molecules. For the alkane-based molecules, βN is usually found around 1.0 per –CH2 for thiol (SH) [23, 38], while around 0.9 and 0.8 per –CH2 are determined for amine (NH2) [23, 31] and carboxylic acid (COOH), respectively . Thus, the tunneling decay with iodine shows the lowest value among those anchoring groups with a trend βN (thiol) > βN (amine) > βN (carboxylic acid) > βN (iodine), which may be due to the difference in the alignment of molecular energy levels to the Fermi level of Au electrode [23, 31]. The tunneling decay of 0.5 per –CH2 can also be converted to 4 nm−1, which is comparable to oligophenyls with 3.5–5 nm−1 [40, 41].
where m is the effective electron mass and is the reduced Planck’s constant. Φ represents the barrier height, which is decided by the energy gap between the Fermi level and the molecular energy levels in the junction. Obviously, the βN value is proportional to the square root of barrier height. Thus, we may propose that iodine-terminated alkane molecules have small Φ with the Au electrode.
Barrier Height of Single Molecular Junctions with Different Anchoring Groups
Taking the –(CH2)6– as the backbone, we performed the rough calculations (see computational detail in Additional file 1) to investigate the frontier molecular orbitals of complexes with four Au atoms at the both ends, including 1,6-hexanedithiol (C6DT), 1,6-hexanediamineb (C6DA), 1,6-hexanedicarboxylic acid (C6DC), and 1,6-hexanediiodo (C6DI). As shown in Table 1, the HOMO and LUMO are − 6.18 and − 1.99 eV, respectively, for C6DT, while HOMO (6.02 eV) and LUMO (− 1.85 eV) are found for C6DA. Meanwhile, HOMO and LUMO energy levels are calculated for C6DC (-6.33 and -2.58 eV) and C6DI (-6.22 and -2.61 eV).
For the Fermi level of Au electrode, we need to consider the influence of the adsorption of molecules. In the vacuum condition, clean Au gives out work function of 5.1 eV ; meanwhile, this value can be obviously changed by the adsorption of molecules. Kim et al.  and Yuan et al.  have found that the work function of Au is around 4.2 eV (4.0–4.4 eV) upon the adsorbed self-assembled monolayers (SAMs) measured by the ultraviolet photoelectron spectrometer (UPS). Low et al. also investigated the electron transport of thiophene-based molecules of TOTOT (LUMO − 3.3 eV, HOMO − 5.2 eV) and TTOpTT (LUMO − 3.6 eV, HOMO − 5.1 eV) with Au as the electrode (T, O, and Op denote thiophene, thiophene-1,1-dioxide, and oxidized thienopyrrolodione, respectively) . The results show that the Fermi level of Au is in the middle of LUMO and HOMO. Thus, we can infer the Fermi level of Au can be around the average energy level of LUMO and HOMO, which are − 4.25 and − 4.35 eV established from TOTOT and TTOPTT, respectively. The Fermi level of Au − 4.25 and − 4.35 eV are similar to that measured by UPS with − 4.2 eV . According to the above, we will use the − 4.2 eV as the Fermi level of Au electrode with the adsorption of molecule.
Assuming the Fermi level of − 4.2 eV for Au with SAM, C6DT and C6DA are the HOMO-dominated electron transport, while LUMO-dominated electron transport is proposed for the C6DC and C6DI. Thus, the barrier height Φ can be established as 1.98 eV (C6DT), 1.82 eV (C6DA), 1.62 eV (C6DC), and 1.59 eV (C6DI) (Table 1). The trend for the barrier height between the molecule and Au is ΦC6DT (thiol) > ΦC6DA (amine) > ΦC6DC (carboxylic acid) > ΦC6DI (iodine), which is consistent with the trend of the tunneling decay (β). Thus, the unusual low tunneling decay can be contributed to the small barrier height between iodine-terminated alkane molecules and Au.
In conclusion, we have measured the conductance of alkane-based molecules with iodine group contacting to Au electrodes by STM-BJ at room temperature. A tunneling decay βN of 0.5 per –CH2 was found for those molecules with Au electrodes, which is much lower than that of alkane-based molecules with other anchoring groups. This can be caused by the small barrier height between the iodine-terminated alkane molecule and Au. The current work shows the important role of the anchoring group in electrical characteristics of single molecular junctions, which can tune the tunneling decay of molecular junction and guide the manufacturing molecular wire.
Highest occupied molecular orbital
Lowest unoccupied molecular orbital
Scanning tunneling microscopy break junction
Ultraviolet photoelectron spectroscopy
Huang CC, Jevric M, Borges A, Olsen ST, Hamill JM, Zheng JT, Yang Y, Rudnev A, Baghernejad M, Broekmann P et al (2017) Single-molecule detection of dihydroazulene photo-thermal reaction using break junction technique. Nat Commun 8:15436
Sedghi G, Garcia-Suarez VM, Esdaile LJ, Anderson HL, Lambert CJ, Martin S, Bethell D, Higgins SJ, Elliott M, Bennett N et al (2011) Long-range electron tunnelling in oligo-porphyrin molecular wires. Nat Nanotechnol 6:517–523
Mativetsky JM, Pace G, Elbing M, Rampi MA, Mayor M, Samori P (2008) Azobenzenes as light-controlled molecular electronic switches in nanoscale metal-molecule-metal junctions. J Am Chem Soc 130:9192–9193
Darwish N, Aragones AC, Darwish T, Ciampi S, Diez-Perez I (2014) Multi-responsive photo- and chemo-electrical single-molecule switches. Nano Lett 14:7064–7070
Zhang JL, Zhong JQ, Lin JD, Hu WP, Wu K, Xu GQ, ATS W, Chen W (2015) Towards single molecule switches. Chem Soc Rev 44:2998–3022
Diez-Perez I, Hihath J, Lee Y, Yu LP, Adamska L, Kozhushner MA, Oleynik II, Tao NJ (2009) Rectification and stability of a single molecular diode with controlled orientation. Nat Chem 1:635–641
Capozzi B, Xia J, Adak O, Dell EJ, Liu Z-F, Taylor JC, Neaton JB, Campos LM, Venkataraman L (2015) Single-molecule diodes with high rectification ratios through environmental control. Nat Nanotechnol 10:522–527
Xu BQ, Xiao XY, Yang XM, Zang L, Tao NJ (2005) Large gate modulation in the current of a room temperature single molecule transistor. J Am Chem Soc 127:2386–2387
Osorio HM, Catarelli S, Cea P, Gluyas JBG, Hartl F, Higgins SJ, Leary E, Low PJ, Martín S, Nichols RJ et al (2015) Electrochemical single-molecule transistors with optimized gate coupling. J Am Chem Soc 137:14319–14328
Perrin ML, Burzuri E, van der Zant HSJ (2015) Single-molecule transistors. Chem Soc Rev 44:902–919
Seth C, Kaliginedi V, Suravarapu S, Reber D, Hong WJ, Wandlowski T, Lafolet F, Broekmann P, Royal G, Venkatramani R (2017) Conductance in a bis-terpyridine based single molecular breadboard circuit. Chem Sci 8:1576–1591
Su TA, Neupane M, Steigerwald ML, Venkataraman L, Nuckolls C (2016) Chemical principles of single-molecule electronics. Nat Rev Mater 1:16002
Yang Y, Liu JY, Feng S, Wen HM, Tian JH, Zheng JT, Schollhorn B, Amatore C, Chen ZN, Tian ZQ (2016) Unexpected current-voltage characteristics of mechanically modulated atomic contacts with the presence of molecular junctions in an electrochemically assisted-MCBJ. Nano Res 9:560–570
Ie Y, Tanaka K, Tashiro A, Lee SK, Testai HR, Yamada R, Tada H, Aso Y (2015) Thiophene-based tripodal anchor units for hole transport in single-molecule junctions with gold electrodes. J Phys Chem Lett 6:3754–3759
Xin N, Jia C, Wang J, Wang S, Li M, Gong Y, Zhang G, Zhu D, Guo X (2017) Thermally activated tunneling transition in a photoswitchable single-molecule electrical junction. J Phys Chem Lett 8:2849–2854
Chen F, Peng LL, Hong ZW, Mao JC, Zheng JF, Shao Y, Niu ZJ, Zhou XS (2016) Comparative study on single-molecule junctions of alkane- and benzene-based molecules with carboxylic acid/aldehyde as the anchoring groups. Nanoscale Res Lett 11:380
Salomon A, Cahen D, Lindsay S, Tomfohr J, Engelkes VB, Frisbie CD (2003) Comparison of electronic transport measurements on organic molecules. Adv Mater 15:1881–1890
Arroyo CR, Tarkuc S, Frisenda R, Seldenthuis JS, Woerde CHM, Eelkema R, Grozema FC, van der Zant HSJ (2013) Signatures of quantum interference effects on charge transport through a single benzene ring. Angew Chem Int Ed 52:3152–3155
Xiang D, Wang X, Jia C, Lee T, Guo X (2016) Molecular-scale electronics: from concept to function. Chem Rev 116:4318–4440
Engelkes VB, Beebe JM, Frisbie CD (2004) Length-dependent transport in molecular junctions based on SAMs of alkanethiols and alkanedithiols: effect of metal work function and applied bias on tunneling efficiency and contact resistance. J Am Chem Soc 126:14287–14296
Leary E, La Rosa A, Gonzalez MT, Rubio-Bollinger G, Agrait N, Martin N (2015) Incorporating single molecules into electrical circuits. The role of the chemical anchoring group. Chem Soc Rev 44:920–942
Capozzi B, Chen Q, Darancet P, Kotiuga M, Buzzeo M, Neaton JB, Nuckolls C, Venkataraman L (2014) Tunable charge transport in single-molecule junctions via electrolytic gating. Nano Lett 14:1400–1404
Chen F, Li XL, Hihath J, Huang ZF, Tao NJ (2006) Effect of anchoring groups on single-molecule conductance: comparative study of thiol-, amine-, and carboxylic-acid-terminated molecules. J Am Chem Soc 128:15874–15881
Kaliginedi V, Rudnev AV, Moreno-Garcia P, Baghernejad M, Huang C, Hong W, Wandlowski T (2014) Promising anchoring groups for single-molecule conductance measurements. Phys Chem Chem Phys 16:23529–23539
Park YS, Whalley AC, Kamenetska M, Steigerwald ML, Hybertsen MS, Nuckolls C, Venkataraman L (2007) Contact chemistry and single-molecule conductance: a comparison of phosphines, methyl sulfides, and amines. J Am Chem Soc 129:15768–15769
Xiang LM, Hines T, Palma JL, Lu XF, Mujica V, Ratner MA, Zhou G, Tao NJ (2016) Non-exponential length dependence of conductance in iodide terminated oligothiophene single-molecule tunneling junctions. J Am Chem Soc 138:679–687
Komoto Y, Fujii S, Hara K, Kiguchi M (2013) Single molecular bridging of Au nanogap using aryl halide molecules. J Phys Chem C 117:24277–24282
Zhou XS, Liu L, Fortgang P, Lefevre A-S, Serra-Muns A, Raouafi N, Amatore C, Mao BW, Maisonhaute E, Schollhorn B (2011) Do molecular conductances correlate with electrochemical rate constants? Experimental insights. J Am Chem Soc 133:7509–7516
Chen L, Wang YH, He B, Nie H, Hu R, Huang F, Qin A, Zhou XS, Zhao Z, Tang BZ (2015) Multichannel conductance of folded single-molecule wires aided by through-space conjugation. Angew Chem Int Ed 54:4231–4235
Mao JC, Peng LL, Li WQ, Chen F, Wang HG, Shao Y, Zhou XS, Zhao XQ, Xie H, Niu ZJ (2017) Influence of molecular structure on contact interaction between thiophene anchoring group and Au electrode. J Phys Chem C 121:1472–1476
Venkataraman L, Klare JE, Tam IW, Nuckolls C, Hybertsen MS, Steigerwald ML (2006) Single-molecule circuits with well-defined molecular conductance. Nano Lett 6:458–462
Peng ZL, Chen ZB, Zhou XY, Sun YY, Liang JH, Niu ZJ, Zhou XS, Mao BW (2012) Single molecule conductance of carboxylic acids contacting Ag and Cu electrodes. J Phys Chem C 116:21699–21705
Huang C, Chen S, Ornso KB, Reber D, Baghernejad M, Fu Y, Wandlowski T, Decurtins S, Hong W, Thygesen KS, Liu S-X (2015) Controlling electrical conductance through a pi-conjugated cruciform molecule by selective anchoring to gold electrodes. Angew Chem Int Ed 54:14304–14307
Kaliginedi V, Moreno-García P, Valkenier H, Hong W, García-Suárez VM, Buiter P, Otten JLH, Hummelen JC, Lambert CJ, Wandlowski T (2012) Correlations between molecular structure and single-junction conductance: a case study with oligo (phenylene-ethynylene)-type wires. J Am Chem Soc 134:5262–5275
Yanson AI, Bollinger GR, van den Brom HE, Agrait N, van Ruitenbeek JM (1998) Formation and manipulation of a metallic wire of single gold atoms. Nat 395:783–785
Eder G, Smith EF, Cebula I, Heckl WM, Beton PH, Lackinger M (2013) Solution preparation of two-dimensional covalently linked networks by polymerization of 1,3,5-tri (4-iodophenyl) benzene on Au (111). ACS Nano 7:3014–3021
Cheng ZL, Skouta R, Vazquez H, Widawsky JR, SchneebeliS CW, Hybertsen MS, Breslow R, Venkataraman L (2011) In situ formation of highly conducting covalent Au-C contacts for single-molecule junctions. Nat Nanotechnol 6:353–357
Li C, Pobelov I, Wandlowski T, Bagrets A, Arnold A, Evers F (2008) Charge transport in single Au/alkanedithiol/Au junctions: coordination geometries and conformational degrees of freedom. J Am Chem Soc 130:318–326
Martin S, Haiss W, Higgins S, Cea P, Lopez MC, Nichols RJ (2008) A comprehensive study of the single molecule conductance of α, ω-dicarboxylic acid-terminated alkanes. J Phys Chem C 112:3941–3948
Wold DJ, Haag R, Rampi MA, Frisbie CD (2002) Distance dependence of electron tunneling through self-assembled monolayers measured by conducting probe atomic force microscopy: unsaturated versus saturated molecular junctions. J Phys Chem B 106:2813–2816
Kim T, Vázquez H, Hybertsen MS, Venkataraman L (2013) Conductance of molecular junctions formed with silver electrodes. Nano Lett 13:3358–3364
Michaelson HB (1977) The work function of the elements and its periodicity. J Appl Phys 48:4729–4733
Kim B, Choi SH, Zhu XY, Frisbie CD (2011) Molecular tunnel junctions based on π-conjugated oligoacene thiols and dithiols between Ag, Au, and Pt contacts: effect of surface linking group and metal work function. J Am Chem Soc 133:19864–19877
Yuan L, Franco C, Crivillers N, Mas-Torrent M, Cao L, Sangeeth CSS, Rovira C, Veciana J, Nijhuis CA (2016) Chemical control over the energy-level alignment in a two-terminal junction. Nat Commun 7:12066
Low JZ, Capozzi B, Cui J, Wei SJ, Venkataraman L, Campos LM (2017) Tuning the polarity of charge carriers using electron deficient thiophenes. Chem Sci 8:3254–3259
We gratefully thank the financial support by the National Natural Science Foundation of China (nos. 21573198, 21273204 and 21406137), Zhejiang Provincial Natural Science Foundation of China (no. LR15B030002), the Natural Science Foundation of Shanghai (no. 17ZR1447100), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, and the Key Laboratory of Spectrochemical Analysis & Instrumentation (Xiamen University), Ministry of Education (SCAI1604).
Availability of Data and Materials
The datasets supporting the conclusions of this article are included within the article and its additional file.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
About this article
Cite this article
Peng, L., Huang, B., Zou, Q. et al. Low Tunneling Decay of Iodine-Terminated Alkane Single-Molecule Junctions. Nanoscale Res Lett 13, 121 (2018). https://doi.org/10.1186/s11671-018-2528-z
- Electron transport
- Barrier height
- Single molecular junction
- Alkyl-based molecules