Halide Anchors for Single-Cluster Electronics

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES8 Aug 2022Halide Anchors for Single-Cluster Electronics Caiyun Wei, Jingyao Ye, Yuming Su, Jueting Zheng, Siqiang Xiao, Jiawei Chen, Saisai Yuan, Chengyang Zhang, Jie Bai, Han Xu, Jia Shi, Jiale Huang and Wenjing Hong Caiyun Wei State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Jingyao Ye State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Yuming Su State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Jueting Zheng State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Siqiang Xiao State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Jiawei Chen State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Saisai Yuan State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Chengyang Zhang State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Jie Bai State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Han Xu State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Jia Shi State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Jiale Huang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 and Wenjing Hong *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 https://doi.org/10.31635/ccschem.022.202202180 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Due to their unique electronic structure, well-defined metal clusters at the atomic level are promising materials for single-cluster electronics. However, coupling between the electrode and the cluster remains challenging mainly due to the coverage of bulky ligands on the noble clusters. Using the scanning tunneling microscopy break junction (STM-BJ) technique, we have developed a "direct contact" approach to fabrication and investigation of the charge transport through single-cluster junctions of AgCu bimetallic metal clusters with different halide anchors. We found that the electrodes could make contact directly with the surface halides of the single-cluster junctions and experience different contact resistance from different halogen atoms. Experiments and calculations reveal that the halide anchors provided efficient coupling between the cluster and the electrode, and the enhanced coupling with various halide anchors promoted electron transport and improved transmission probability. Our work offers a "direct contact" strategy for interface design between clusters of noble metals and electrodes, an essential step in progress toward single-cluster electronics. Download figure Download PowerPoint Introduction Atomically precise metal clusters have unique properties such as optical absorption,1 chiral,2 magnetism,3 and catalytic activity;4 they are considered promising functional materials for fabrication of single-cluster electronic devices,5 including single-electron transistors,6,7 and field-effect transistors.8 In the fabrication of single-cluster electronic devices, the cluster-electrode interface plays an essential role in affecting the binding energy and the electronic coupling between molecules and electrodes.9,10 However, precise design and fabrication of the anchoring sites between the electrode and cluster remain challenging as a result of the hindrance of the noble metal cluster by bulky ligands.11,12 The fabrication of a well-defined cluster-electrode interface is the most critical step in assembling the cluster into single-cluster electronics.13 Pioneering studies of single-cluster electronic devices defined the contact sites for the cluster-electrode interface by attaching the anchoring sites to the different ligands to produce different clusters.14–22 One end of the ligand was coordinated with the clusters, and the other end acted as the anchoring group between the cluster cores and the electrodes. However, ligands with two terminals also increased the contact resistance with the cluster core, and chalcogenide atoms used as anchoring sites on single cobalt chalcogenide cluster junctions led to a multitude of sites for cluster/electrode binding (Scheme 1), which limited the feasibility of cluster structures for single-cluster electronics.22 Recent advances suggest that the halides, via their lone electron pairs, could serve as anchoring sites for single-molecule junctions23 and even perovskite junctions,24 thereby offering a new opportunity for the direct contact of the electrode with halogen atoms on the cluster surfaces and more efficient electrode-cluster coupling. Scheme 1 | Illustration of single-cluster junction motifs of different atomically well-defined metal clusters. POMs, polyoxometalates. Download figure Download PowerPoint In this work, using the scanning tunneling microscopy break junction (STM-BJ) technique,25 we have developed a "direct contact" approach to fabricate the single-cluster junctions of AgCu clusters with three halide anchors. AgCu bimetallic metal clusters were designed and synthesized with halogens to stabilize the structure. The halogens with lone electron pairs could also bind to coordinatively unsaturated Au atoms. The single-cluster conductance measurements revealed that each cluster device showed three conductance features, with decreasing order of conductance, as follows: AgCuCl < AgCuBr < AgCuI. Analysis of electrostatic potentials (ESP) and average local ionization energy (ALIE) found that the negative ESP and the lowest values of ALIE were around the halogen exposure sites and the surrounding metals, suggesting that halides could serve as the anchoring sites. This was also confirmed by Raman spectroscopy and current-voltage measurements. Tight-binding (TB) calculations demonstrated that the transmission of the single-cluster depended on the coupling between the halide anchors and gold electrodes, and enhanced coupling with various halide anchors increased the transmission probability. Experimental Methods Synthesis of AgCuX AgSbF6 (0.1 mmol) was added to a mixture of 4-(tert-butyl)phenylacetylene (4-tBuPA, 0.1 mmol) and 1,5-Bis(diphenylphosphino)pentane (dpppe, 44 mg, 0.1 mmol) in ethanol (EtOH, 3 mL) and N, N-dimethylformamide (DMF, 2 mL). While stirring, CuX (0.04 mmol, X = Cl, Br, and I) was added. The reaction mixture was stirred at room temperature (rt) for 1 day; then the resulting mixture was centrifuged for 3 min at 12,000 rpm. After filtration, the filtrate was evaporated at 4 °C to afford orange crystals after 15 days. Conductance and current–voltage measurements An STM-BJ26,27 technique measured the current through a cluster trapped between an Au tip and the gold substrate. The Au tip was made from gold wire (99.999%, 0.25 mm diameter). The substrates used Au(111) films on glass wafers. All of the clusters were dissolved in 99.9% propylene carbonate (4-methyl-1,3-dioxolan-2-one, PC, Sigma-Aldrich, Shanghai, China) to prepare a 0.1 mM solution. The gold wire was etched in a solution of 37% HCl∶EtOH (1∶1,v/v) at a constant potential of 2.3 V to form a sharp shape, which was then coated with Apiezon wax. A constant bias potential (V bias) of 100 mV was applied between the gold substrate and the gold tip. The experiments were carried out at rt in the air. A gold tip moved up and down to touch the gold substrate, leading to the continual formation and breaking of the junction. When the gold tip moved up, a single gold atom junction (the conductance quantum G0) was formed due to the ductile nature of the gold. When it continued to open, the Au–Au contact fracture formed a nanogap between the tip and the substrate; then the cluster entered the gap to form a single cluster junction. Thousands of single-cluster junctions were constructed by moving the tip up and down repeatedly, while the current versus length traces were recorded. The distance between the tip and substrate was the sum of the length of conductance junction, and an Au–Au snap back distance (0.5 nm). The characterization of the current–voltage (I–V) of a cluster junction was based on previously published methods.19 When a low conductance plateau was observed to have been formed at 0.1 V bias, the movement of the gold tip was stopped, and the bias voltage was scanned between −1 and 1 V, leading to I–V traces. The 2D I–V uses Gaussian fitting in each array column to obtain the most probable I–V curve. Electronic coupling strength across the molecule-halide interface (Γ) was obtained by fitting the I–V responses to the theoretical model.28 Theoretical calculation Density functional theory (DFT) calculations of the metal clusters were performed with the quantum chemistry program Gaussian09.29 The geometries of the metal clusters were obtained from structures derived from single-crystal X-ray diffraction (SCXRD); the position of metals and ligands were fixed and optimized for the remainder of the atoms. Due to the presence of metals, the def2-SVP,30 double -ζ valence effective core potential (ECP) basis set was used for the metal clusters. To decrease computation cost, we employed two classical DFT functions and PBE0 for metal clusters. The wave function analysis was accomplished by Multiwfn3.831 at the same level as the program utilized for geometry optimization with the default settings. ESP and ALIE results were generated by calculating grid data of the mapped function and exporting it using Multiwfn.32 During these analyses, the van der Waals (vdW) surface denoted an isosurface of ρ = 0.001 e/bohr3. All the ESP maps and ALIE maps were produced and rendered by the visual molecular dynamics (VMD) 1.9.4 program.33 To determine the electron transport properties of the clusters, we used the TB method to theoretically and qualitatively estimate the conductance variation with different halides. In view of the coupling strength values in Table 1, we used 0.8, 1.6, and 2.4 as the coupling constant (ΓTB) between the gold electrode and each Cl, Br, and I anchoring site. The transmission was calculated from the equation: T ( E ) = T r [ Γ L G r Γ R G a ] where ΓL is the coupling matrix to the left electrode and ΓR to the right electrode. Gr and Ga correspond to the retarded and advanced Green's functions.34 Table 1 | The ALIE Around Three Halides on a Different Surface, the Coupling Strength (Γ), Raman Shift, HOMO–LUMO Gap, and Conductance of Three Clusters Cluster ALIE (eV) Γ (eV) Raman Shift (cm−1) ΔEH–L (eV) Conductance (log G/G0) AgCuCl 8.56 ΓL = 0.00285 ΓR = 0.00288 224 2.919 −4.30 8.60 8.64 AgCuBr 7.89 ΓL = 0.00443 ΓR = 0.00497 228 2.954 −3.97 7.93 8.04 AgCuCl 7.24 ΓL = 0.00772 ΓR = 0.00795 231 2.947 −3.73 7.68 7.76 Results and Discussion Design, synthesis, and characterization of the clusters Stabilization of the structure of clusters by halogens has been studied in the Au,35 Ag,36 and AuAg37 clusters both experimentally and theoretically.38 Zhu et al.39 have reported that chloride could exist inside the structure as a bridge connecting the kernel and the outer shell. Based on previous studies, three alloy clusters with halide ligands, [Ag12Cu7(4-tBuPhC≡C)14(dpppe)3X3](SbF6)2 (X = Cl, Br, and I), termed AgCuCl, AgCuBr, and AgCuI, respectively, were designed and synthesized in a mixture of Ag and Cu compounds (Figure 1a). The atomically precise structures were determined by SCXRD and electrospray ionization-mass spectroscopy (ESI-MS) ( Supporting Information Figure S1, Table S1). Figure 1 | Synthetic and X-ray crystallographic structure of clusters. (a) Synthetic routes for AgCuX. (b) Centered M13 in [Ag12Cu7(4-t-Bu-PhC≡C)14(dpppe)3X3]2+ (X = Cl, Br, and I). (c) The core of [Ag12Cu7(4-t-Bu-PhC≡C)14(dpppe)3X3]2+. Color codes: cyan, Ag; orange, Cu; red, halogen; gray, H atoms, and anions have been omitted for clarity. Download figure Download PowerPoint For the three clusters with similar mixed ligands and metals, the core structures were shown to be precise. In the cluster AgCuX (Figure 1c, X = Cl, Br, and I), the heterometallic cluster complex consisted of 12 Ag, 7 Cu, and 3 halide atoms encircled by 14 rigid 4-tBuPA with bulky substituent groups and 3 dpppe. The P in the cluster was generally monosubstituted. The three auxiliary diphenylphosphine ligands stabilized the Ag12Cu7X3 cluster structure remarkably. The center structure of Ag12Cu7X3 is an M13 core consisting of a Cu atom and three Ag2Cu2 (Figure 1b), and the M13 core was extended by three tetrahedral Ag2X (X = Cl, Br, and I) motifs with a C3 symmetry axis through the central Cu atom (Figure 1c). The core was similar to a previously reported cluster [Ag16Cu9(μ-DPEPP)3(C≡CC6H4-t-Bu-4)20]5+ (DPEPP = bis(2-(diphenylphosphino)ethyl)-phenylphosphine).40 The dpppe ligand adopted a bridging mode with two P donors bound to two silver centers, forming 11-membered coordination rings. The acetylides exhibited two types of asymmetric bonding modes with Ag and Cu, including μ3-η1(σ):η1(σ):η2(π) and μ4-η1(σ):η1(σ):η1(σ):η2(π) ( Supporting Information Figure S2). The halides were located in the three corners of clusters with a similar chemical environment. The Ag-X-Ag (X = Cl, Br, and I) bridge connects the Cu atoms at the M13 core to sustain the overall structural stability, resulting in three unprecedented AgCu clusters. Halides localized inside the cluster structure as their sizes fitted the distance of the two Ag on the top of corners; similar behaviors were exhibited with thiolates as peripheral ligands.38 The halides played a crucial role in the structure of three clusters because they connected the two Ag atoms, which extended from the metal core via Ag–X bond. Single-cluster conductance measurements The conductance measurements for the three clusters were carried out using STM-BJ with 0.1 V bias dissolved in propylene carbonate at rt.41 Figure 2a displays the one-dimensional (1D) conductance histograms in the logarithmic scale from statistical analysis and typical conductance-displacement traces (Figure 2b, Supporting Information Figure S7) of AgCuCl (green), AgCuBr (tangerine), and AgCuI (violet). Each cluster had three prominent conductance peaks, with the most apparent conductance at 10−4.30±0.12G0 (3.88 nS, AgCuCl), 10−3.97±0.9G0 (8.30 nS, AgCuBr), and 10−3.73±0.7G0 (14.43 nS, AgCuI). The low conductance peaks decreased in the following sequence: AgCuCl < AgCuBr < AgCuI, indicating that a similar structure with different halides could affect the charge transport capacity significantly. Figure 2 | Conductance measurement of clusters. (a) 1D Conductance histogram of AgCuCl (green), AgCuBr (tangerine), and AgCuI (violet). (b) Typical conductance-displacement traces of AgCuCl (green), AgCuBr (tangerine), and AgCuI (violet). (c) 2D conductance histogram of AgCuCl (green), AgCuBr (tangerine), and AgCuI (violet). (d–f) The relative stretching displacement histograms and the theoretical distance of anchoring sites predicted for AgCuCl (green), AgCuBr (tangerine), and AgCuI (violet) (inset). Download figure Download PowerPoint To investigate the anchoring sites, 2D conductance histograms were constructed to obtain the stretching distance distributions. The actual stretching distance of the cluster is the sum of the Au–Au snap back distance (0.5 nm) and the statistical length.42 As shown in Figure 2d–f, for AgCuX, the most probable length of the molecular junction of each conductance feature was 0.65 ± 0.02 nm, and the actual junction length for each conductance was about 1.15 ± 0.02 nm. The 2D conductance-displacement histograms of AgCuX (Figure 2c) showed similar stretching distances as the related structures. We also determined from theoretical simulations in the crystal structure that the mean distance between different sites of AgCuCl, AgCuBr, and AgCuI is about 1.10 ± 0.05 nm (Figure 2d–f, inset). The relative stretching distance suggests that the anchoring sites were possibly located around the halogen atoms. An earlier study of single-cluster junctions with cobalt chalcogenides revealed that the multiple binding sites would lead to different conductance and, ultimately, a broad conductance distribution due to several possible charge transport pathways among the anchoring sites.22 However, the single-cluster junctions with three halides possess a dominant and well-defined conductance due to their high symmetry and a similar chemical environment with respect to the three halogen atoms. The 2D conductance histograms and positively correlated region centered at the conductances also revealed the three conductance origins resulting from the movement of tips anchored at different sites on the surface of clusters of the conductance ( Supporting Information Figure S5). The other anchoring sites might be the metals around the halogen atoms. We also analyzed the ligands of this cluster in control experiments designed to exclude the ligands' conductance signals ( Supporting Information Figure S3). A sharp conductance from the ligands shaded by the cluster was not found, confirming that the clusters were stable during the conductance measurements ( Supporting Information Figure S4). To determine the coupling stretch further, I–V measurements were performed for the three cluster junctions over a bias range of ±1 V. As shown in the 2D I–V histogram in Figure 3a–c, three cluster junctions had similar curves with higher symmetric characteristics at the positive and negative polarities. The I–V curves preclude a two-step transport process (through ligands to the metal core), like electron tunneling through a double-barrier in the single-cluster junctions,19 whereby electrons first move through the ligand from the cluster core and then to the other electrode. Electronic coupling strength across the molecule-halides interfaces (Γ) can be obtained by fitting the I–V responses to a single-level tunneling transport model. Assuming a single-level tunneling process, the I–V curves were fitted to the following theoretical model43–45 ( Supporting Information Figure S9): I V = 8 e h Γ L Γ R Γ L + Γ R { tan − 1 Γ R Γ L + Γ R e V − ε 0 Γ L + Γ R + tan − 1 Γ L Γ L + Γ R e V + ε 0 Γ L + Γ R } where ε0 is the energy of the molecular orbital involved in the charge transfer process, ΓL and ΓR are the coupling strength between the molecular wire and both left and right electrodes, respectively.45,46 The calculated coupling strength (Γ) of the clusters is entered in Table 1 and indicates that the coupling of three clusters follows the order AgCuI > AgCuBr > AgCuCl. We also performed the surface-enhanced Raman spectroscopy (SERS) to provide more evidence of the cluster and the Au–halides bond, as shown in Figure 3d and Supporting Information Figure S8. The Raman peaks were observed at approximately 224, 228, and 231 cm−1 for AgCuCl, AgCuBr, and AgCuI, respectively, consistent with the formation of Au–Cl, Au–Br,47 and Au–I bonds, also proving cluster and electrode binding via Au–halogen bonds, with the binding strength following the order of Au–Cl < Au–Br < Au–I.24,48 Figure 3 | Current–voltage measurement of AgCuCl (a), AgCuBr (b), and AgCuI (c). The gray line represents the most probable curve of conductance-voltage from the Gaussian fitting. (d) Raman spectra of Au–halides interaction on the gold substrates with Au nanoparticles. Download figure Download PowerPoint Theoretical analysis of anchoring sites To further determine whether the clusters could interact with electrodes, DFT calculations of the metal clusters were performed.31 As reported by Stenlid and Brinck,49 because the valence configuration (d10s1) of Au is similar to that of hydrogen in Au tips without ligands, there is a singly occupied s-orbital, and positive ESP maps surrounding low coordinated Au the of the surface ESP were located in the and the surface electrostatic in the corners > > the anchoring groups be the of the negative However, using ESP to the anchoring sites results in several For the ESP is it could not bind to the Au tips due to the to bind The of ALIE analysis and ESP has been used to the of sites, and binding sites on suggesting that the approach could provide a unique to the anchoring sites of the clusters. An of the atomically precise clusters was performed to the anchoring sites of the single-cluster junctions (Figure 4 and Supporting Information and As in Figure the ESP and ALIE surface and the values of ESP and ALIE surrounding halides are also The of the structure was in the same as the ESP and ALIE maps to the anchoring sites and the of the structure that the halides are and and not by the bulky ligands, as shown in Figure As shown in Figure a negative ESP developed around the and the halogen atoms to the lone of the lowest values of ALIE were found around the halogens (Figure The structure also metal atoms around the halides, where a negative potential Based on the we a to the process of forming a single-cluster junction. When the two electrodes are the Au tips and the halides on the cluster are by electrostatic by the When the distance between is the and the single-cluster junction by are the interaction of suggests that the Au tips the the and could not with the Au tips because the bound As shown in Figure the ALIE the halide group follows the of AgCuI < AgCuBr < AgCuCl, the of the coupling strength between the Au tips and the anchoring atoms follows a Figure 4 | ESP maps and ALIE maps on the surface of clusters. The ESP of (a), AgCuCl, (b) AgCuBr, and (c) AgCuI. (d–f) The ALIE maps on the surface of (d) AgCuCl, AgCuBr, and AgCuI. The of AgCuCl, AgCuBr, and AgCuI. Download figure Download PowerPoint The distance between the anchoring predicted by cluster with the experimentally determined junction suggesting that the electrodes binding to the clusters were via the halides (Figure This could be by the relative stretching displacement histograms ( Supporting Information Figure and theoretical calculations with TB ( Supporting Information Figure which that the of the three conductances in the 2D conductance histogram junction might be by the between the electrodes and the halides As shown in Supporting Information Figure the two electrodes were anchored in different metals around halides, a higher conductance When of the electrodes was to the and was to the halides, a conductance was The low conductance was by both left and right with the halides. in the electronic coupling (Γ) also with the ESP and ALIE in the coupling strength between the two of the cluster might be by of ligands around different halides. This was further confirmed by the different ALIE values obtained for different halides on the same cluster surface of charge transport single-cluster junctions To further the between the structure and charge transport through these clusters, an analysis of was used to reveal the of conductance As shown in Supporting Information Figure the occupied molecular orbital was localized mainly on the ligands, including the while the lowest molecular orbital was in the metal As shown in the average transmission spectra for different AgCu clusters (Figure with the TB the HOMO–LUMO gap of single-cluster junction decreased with AgCuCl, AgCuBr, and AgCuI, while the transmission function increased The coupling between the cluster and the ligands promoted electron transport and increased the transmission probability. the HOMO–LUMO gap showed a the anchoring strength of the AgCuI consistent with the Figure | (a) The of the single-cluster junction. (b) The average transmission spectra for AgCuCl (green), AgCuBr (tangerine), and AgCuI (violet) are anchored with halides. Download figure Download PowerPoint To reveal the role of the different halogen anchors on the charge the between the ALIE values the the HOMO–LUMO the coupling strength (Γ), and the Raman and the conductance of the three clusters are shown in Table The three clusters had similar structures and distances between the anchoring sites the electronic structure and would affect the charge transport at single-cluster Table 1 that the clusters have a similar structure different anchoring sites, the coupling strength is the conductance and is more the HOMO–LUMO we found that the the binding of the anchoring and electrode, the higher the We have developed a "direct contact" approach for a of AgCu bimetallic metal clusters with three halides as anchoring sites. We synthesized a of AgCu metal clusters with three halides and, using both and theoretical the halides as the anchoring sites in single-cluster Raman and I–V measurements provided direct evidence that halides could serve as the anchoring sites, and the coupling strength increased in the order < < A theoretical calculation with the TB method also that the conductances of clusters decreased in the following sequence: AgCuCl < AgCuBr < AgCuI the electrodes were anchored with halides. Our results that the surface halides could be the anchoring ligands to fabricate single-cluster offering a new strategy for clusters into Supporting Information Supporting Information is and materials and control

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