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 −1 exponentially with the increase of molecular length with a decay constant β =0.5 per –CH (or 4 nm ). Importantly, the N 2 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. Keywords: Electron transport, Barrier height, Single molecular junction, Iodine, Alkyl-based molecules Background possible to tune the molecular energy level towards the Understanding the electron transport of single-molecule Fermi level to achieve the low decay [21–24]. junctions is crucial for the development of molecular In single-molecule junctions, the anchoring group electronic devices [1–16]. The non-resonant tunneling plays an important role in the control of electronic model has often been used to describe the electron coupling between the organic backbones with the transport process through small molecule, where contact electrodes [21, 23–25]. A series of conductance mea- conductance, molecular length, and the tunneling decay surements for the alkane-based molecules have showed constant are the main parameters [17, 18]. In most mo- a significant effect of different anchoring groups on the lecular systems, decay constant is highly related to the binding geometry, junction formation probabilities, con- electronic properties of organic backbone. For example, tact conductance, and even conductance channel the conjugated molecular systems have low tunneling (through LUMO or HOMO) of molecular junctions decay, unlike non-conjugated ones [17, 19]. Since the [21–25]. Since the anchoring group can regulate the tunneling decay is decided by the barrier height between frontier orbitals in the molecular junction, the tunneling the Fermi level of electrode and lowest unoccupied mo- decay of the molecule may also be tuned by the ancho- lecular orbital (LUMO) or highest occupied molecular ring group . However, limited study has been orbital (HOMO) of molecular junctions [17, 20], it is 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- * Correspondence: email@example.com; firstname.lastname@example.org; BJ) (Fig. 1)[26, 27]. The single molecular con- email@example.com ductance measurements show that the conductance Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, decreases exponentially with the increase of molecular Zhejiang, China lengths and the decay constant of alkane molecules Department of Applied Chemistry, Zhejiang Gongshang University, with iodine group is much lower than that of the Hangzhou 310018, China Shanghai Key Laboratory of Materials Protection and Advanced Materials in analogues with other anchoring groups. The different Electric Power, Shanghai University of Electric Power, Shanghai 200090, China © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Peng et al. Nanoscale Research Letters (2018) 13:121 Page 2 of 6 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 Fig. 1 Schematic diagram of scanning tunneling microscopy on Au(111) with monolayer of 1,4-butanediiodo by break junction (STM-BJ) and molecular structures. a Schematic of the STM-BJ with molecular junction. b Molecular structures of STM-BJ. Figure 2a gives out the typical conductance alkane iodine molecules traces exhibiting the stepwise feature. Conductance traces show plateau at 1 G , indicating the formation of stable Au atomic contact. Plateau at a conductance value −3.6 tunneling decay constants for alkane molecules with of 10 G (19.47 ns) is also found besides the 1 G , 0 0 varied anchoring groups are explained by barrier owing to the formation of molecular junction. A height between molecule and electrode. conductance histogram could also be obtained by treating with logarithm and binning of conductance Methods value from more than 3000 conductance traces, and 1,4-Butanediiodo, 1,5-pentanediiodo, and 1,6-hexane- then, the intensity of conductance histogram was diiodo were purchased from Alfa Aesar. All solutions normalized by the number of traces used and shows a −3.6 were prepared with ethanol. Au(111) was used as the conductance peak at 10 G (19.44 ns) (Fig. 2b). Those substrate, while mechanically cut Au tips were used as show that the iodine group can serve as an effective the tips. Before each experiment, the Au(111) was anchoring group forming molecular junction. However, electrochemically polished and carefully annealed in a this value is smaller than the single molecular butane flame and then dried with nitrogen. conductance value of 1,4-butanediamine with amine as The Au(111) substrate was immersed into a freshly the anchoring group, which may stem from weak prepared ethanol solution containing 0.1 mM target interaction between iodine and Au electrode . molecules for 10 min. The conductance measurement In comparison with 1,4-diiodobutane, pronounced −3.8 −4.0 was carried out on the modified Nanoscope IIIa STM peaks at 10 G (12.28 ns) and 10 G (7.75 ns) are 0 0 (Veeco, USA.) by using the STM-BJ method at room found for 1,5-pentanediiodo and 1,6-hexanediiodo, temperature [28–30], which simply measured the con- respectively (Fig. 3). The conductance values decrease ductance of single-molecule junctions formed by with the increasing of molecule length. Meanwhile, the Fig. 2 Single molecular conductance of Au–1,4-butanediiodo–Au junctions. a Typical conductance curves of Au–1,4-butanediiodo–Au junctions measured at a bias of 100 mV. b Log-scale conductance histogram of 1,4-butanediiodo junctions with Au contacts Peng et al. Nanoscale Research Letters (2018) 13:121 Page 3 of 6 Fig. 3 Single molecular conductance of 1,5-pentanediiodo and 1,6-hexanediiodo with Au electrode. Log-scale conductance histogram of single molecular junctions with a 1,5-pentanediiodo and b 1,6-hexanediiodo conductance values of 1,5-pentanediiodo and 1,6- distances are comparable to the length of molecules. hexanediiodo are smaller than those of 1,5- Eder et al. reported that the adsorption of 1,3,5-tri pentanediamine and 1,6-hexanediamine, respectively (4-iodophenyl)-benzene monolayer onto Au(111) may , which may be caused by the different interaction in cause partial dehalogenation ; however, a very alkane-based molecular junctions between iodine and larger conductance value for those Au–C covalent amine anchoring groups binding to Au electrodes . contact molecular junctions can be found for −1 The two-dimensional conductance histograms were also molecules with four (around 10 G )and six (bigger −2 constructed for those molecular junctions (Additional file 1: than 10 G ) –CH – units . Thus, we propose 0 2 Figure S1) and give out similar conductance values of that the current investigated molecules contact to the one-dimensional histograms. Typically, the breaking off Au through the Au–I contact. distance of molecular junctions increases with the increas- ing of molecular length. We also analyze the distance from Tunneling Decay Constant of Iodine-Terminated Alkane −5.0 −0.3 the conductance value of 10 G to 10 G as shown Single Molecular Junctions 0 0 in Fig. 4, and rupture distances of 0.1, 0.2, and 0.3 nm are Under the current bias, those molecule conductance can found for 1,4-butanediiodo, 1,5-pentanediiodo, and 1,6- be expressed as G = Gc exp(–β N). Here, G is the hexanediiodo, respectively. Here, the rupture distances are conductance of the molecule and Gc is the contact con- obtained from the maximum peak of the rupture distance ductance and is determined by the interaction between histogram . It was reported that there is a snap back the anchoring group and the electrode. N is the methy- distance of 0.5 nm for Au after the breaking of Au–Au lene number in the molecule, and β is the tunneling contact [34, 35]; thus, the absolute distances for those decay constant, which reflects the coupling efficiency of molecular junctions between electrodes could be 0.6, 0.7, electron transport between the molecule and the elec- and 0.8 nm which are found for 1,4-butanediiodo, 1,5- trode. As show in Fig. 5, we plot a natural logarithm pentanediiodo, and 1,6-hexanediiodo, respectively. Those scale of conductance against the number of methylene; Fig. 4 Breaking off distances for iodine-terminated alkanes. Breaking off distances of a 1,4-butanediiodo, b 1,5-pentanediiodo, and c 1,6-hexane- −5.0 −0.3 diiodo obtained from conductance curves between 10 G and 10 G 0 0 Peng et al. Nanoscale Research Letters (2018) 13:121 Page 4 of 6 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-hexane- diamineb (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, respect- ively, 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 con- sider 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 Fig. 5 Single-molecule conductance vs molecular length for of Au is around 4.2 eV (4.0–4.4 eV) upon the adsorbed iodine-terminated alkanes. Logarithmic plots of single-molecule self-assembled monolayers (SAMs) measured by the conductance vs molecular length for iodine-terminated alkanes ultraviolet photoelectron spectrometer (UPS). Low et al. also investigated the electron transport of thiophene- tunneling decay constant β of 0.5 per –CH is deter- based molecules of TOTOT (LUMO − 3.3 eV, HOMO N 2 mined from the slope of linear fitting. This tunneling − 5.2 eV) and TTO TT (LUMO − 3.6 eV, HOMO − 5. decay is very low in alkane-based molecules. For the 1 eV) with Au as the electrode (T, O, and O denote alkane-based molecules, β is usually found around 1.0 thiophene, thiophene-1,1-dioxide, and oxidized thieno- per –CH for thiol (SH) [23, 38], while around 0.9 and pyrrolodione, respectively) . The results show that 0.8 per –CH are determined for amine (NH )[23, 31] the Fermi level of Au is in the middle of LUMO and 2 2 and carboxylic acid (COOH), respectively . Thus, the HOMO. Thus, we can infer the Fermi level of Au can be tunneling decay with iodine shows the lowest value around the average energy level of LUMO and HOMO, among those anchoring groups with a trend β (thiol) > which are − 4.25 and − 4.35 eV established from β (amine) > β (carboxylic acid) > β (iodine), which TOTOT and TTO TT, respectively. The Fermi level of N N N P may be due to the difference in the alignment of mo- Au − 4.25 and − 4.35 eV are similar to that measured by lecular energy levels to the Fermi level of Au electrode UPS with − 4.2 eV . According to the above, we will [23, 31]. The tunneling decay of 0.5 per –CH can also use the − 4.2 eV as the Fermi level of Au electrode with −1 be converted to 4 nm , which is comparable to the adsorption of molecule. −1 oligophenyls with 3.5–5nm [40, 41]. Assuming the Fermi level of − 4.2 eV for Au with SAM, The β for the metal-molecule-metal junctions can be C6DT and C6DA are the HOMO-dominated electron simply described by the below equation [17, 20, 38], transport, while LUMO-dominated electron transport is proposed for the C6DC and C6DI. Thus, the barrier rﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ height Φ can be established as 1.98 eV (C6DT), 1.82 eV 2mΦ β α (C6DA), 1.62 eV (C6DC), and 1.59 eV (C6DI) (Table 1). The trend for the barrier height between the molecule and where m is the effective electron mass and is the re- Au is Φ (thiol) > Φ (amine) > Φ (carboxylic C6DT C6DA C6DC duced Planck’s constant. Φ represents the barrier height, acid) > Φ (iodine), which is consistent with the trend C6DI which is decided by the energy gap between the Fermi Table 1 Energy levels of the frontier orbitals of molecules level and the molecular energy levels in the junction. contacting with four Au atoms computed by DFT method Obviously, the β value is proportional to the square Au -C6DT-Au Au -C6DA-Au Au -C6DC-Au Au -C6DI-Au 4 4 4 4 4 4 4 4 root of barrier height. Thus, we may propose that (eV) (eV) (eV) (eV) iodine-terminated alkane molecules have small Φ with E − 1.99 − 1.85 − 2.58 − 2.61 LUMO the Au electrode. E − 6.18 − 6.02 − 6.33 − 6.22 HOMO E - 2.21 2.35 1.62 1.59 Barrier Height of Single Molecular Junctions with LUMO Au Different Anchoring Groups E - 1.98 1.82 2.13 2.02 Au Taking the –(CH ) – as the backbone, we performed the 2 6 HOMO rough calculations (see computational detail in Peng et al. Nanoscale Research Letters (2018) 13:121 Page 5 of 6 of the tunneling decay (β). 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Published: Apr 24, 2018
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