Letter pubs.acs.org/Langmuir
Nanoscale Origin of Defects at Metal/Molecule Engineered Interfaces Peter N. Nirmalraj,* Heinz Schmid, Bernd Gotsmann, and Heike Riel IBM ResearchZurich, Säumerstrasse 4, CH-8803 Rüschlikon, Switzerland S Supporting Information *
ABSTRACT: The control and repair of defects at metal/molecule interfaces is central to the realization of molecular electronic circuits with reproducible performance. The fundamental mechanism governing defect (pore) evolution on mica-supported metal surfaces, its propagation in self-assembled molecular layers, and its implications for molecular junction devices are discussed. Pore eradication by replacing mica with halide platforms coupled with elevated substrate temperature during metal deposition yields exceptionally ultraflat metal landscapes. In situ scanning tunneling microscopy further substantiates molecular locking at defect sites and upon defect healing; the emergence of a closely packed 2-D molecular architecture is demonstrated with nanometer-scale spatial resolution in liquids.
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support structure because of its unique smooth surface and minimal surface treatment prior to metal deposition. In addition to mica, various substrates such as hydrogenpassivated,19 surface-modified silicon,20 and glass slides21,22 have also been employed as support substrates for molecular adsorption studies. Multistep techniques such as template stripping,23 solid-state bonding,24 flame annealing,25 vacuum annealing,26 template stripping within solvents,27 liquid-glass template stripping,28 and a replica technique29 have produced gold contacts with grain sizes from 25 nm to 1 μm14,23,25,27 and a mean surface roughness ranging from 2 to 25 Å14,23,26,27,30,31 over an area of several micrometers. In spite of the vast body of literature available on the subject of forming ultraflat and defect-free gold contacts, very few have addressed the subject of pore formation11,32−34 occurring on gold surfaces. Fewer still have emphasized its role in the quality of molecular layers formed on such metal contact surfaces. In this letter, we elucidate the underlying mechanism behind pore formation and the influence of substrate temperature during metal deposition on the pore density and geometry. We highlight the occurrence of pinholes in C60 monolayers originating from the imperfections in the underlying gold contact using high-resolution in situ scanning tunneling microscopy (in situ STM).35 Furthermore, we have demon-
nderstanding the origin of defects at electronically engineered interfaces between molecular ensembles and metal contacts is crucial to controlling interfacial charge transfer in molecular electronic circuits. Of particular interest are defects occurring at metal−molecule−metal (M−m−M) junction interfaces based on self-assembled monolayers (SAMs) of tailored organic molecules1−3 sandwiched between metal contacts. These M−m−M architectures with tunable electronic properties4 based on a relatively simple device fabrication approach provide a promising route to developing hybrid molecular electronic devices.5,6 However, there remain certain outstanding challenges in constructing robust M−m−M junctions, such as fabricating a defect-free ultraflat (bottom) contact on which the molecules can self-assemble and adding a reliable (top) contact5,7 without compromising the device functionality. Here we focus on the evaluation of a bottom contact that leads to defect-free SAMs, which is a requirement for building stable, reproducible molecular electronic devices. Previous studies have focused on fabricating ultraflat gold contacts for applications ranging from plasmonics8 to protein chip development.9 Although gold is a popular metal contact of choice owing to its chemical inertness and biocompatibility, it is still subject to numerous topological imperfections. Earlier reports have investigated the influence of the substrate material,10 temperature,11 deposition rate,12 and chamber pressure during evaporation11,13 on the grain size, in-grain root-mean-square roughness (rms), and surface defects. A vast majority of such studies, however, employ mica14−18 as the © 2013 American Chemical Society
Received: November 19, 2012 Revised: January 11, 2013 Published: January 22, 2013 1340
dx.doi.org/10.1021/la3046109 | Langmuir 2013, 29, 1340−1345
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Letter
Figure 1. AFM analysis of an ultrathin gold surface as a function of substrate (mica) temperature during metal deposition. (a) 10 × 10 μm2 (z range is 20 nm) topographic map of Au(111) on a mica substrate held at ∼350 °C during metal deposition with a mean grain size of 650 ± 150 nm and a pore density of 50 ± 10 pores per 10 μm2. (b) Corresponding 3D map of the pore depth profile within the Au metal surface. (c) 10 × 10 μm2 (z range is 20 nm) topographic map of Au(111) on a mica substrate held at ∼470 °C during metal deposition showing the reduction in pore density of 12 ± 4 pores per 10 μm2 and increase in mean grain size of 1 ± 0.2 μm. (d) Corresponding 3D map of the topographic data shown in (c). (e, f) Analysis of the pore density and grain size as a function of the substrate temperature, respectively.
AFM topographic data, which is actually lower than the pore density values reported earlier32 for Au(111) surfaces on silicon substrates. A 3D map of the gold surface reveals pores with varying depth seen in the AFM image (Figure 1b). A mean pore depth of 55 ± 35 nm is calculated from the section analysis of various pores. Note that the measured depth of the pores is a lower bound because measuring the actual depth is limited by the geometry of the AFM probe and imaging conditions. A relatively homogeneous AFM topographic map of a gold surface with a smaller population of pores is seen in Figure 1c. In comparison to Figure 1a, this surface was formed by evaporating gold onto a mica substrate held at ∼470 °C. We observed an increase in the mean grain size of 1 ± 0.2 μm and a striking reduction in the mean pore density of 12 ± 4 pores per 10 μm2 upon elevating the substrate temperature during deposition. From the corresponding 3D map (Figure 1d), a reduction in the mean pore depth to 8 ± 3 nm is also calculated. To obtain the optimum temperature, we prepared numerous samples under exactly the same deposition conditions but varied only the deposition temperature and analyzed the influence of the elevated substrate temperature on the pore density (Figure 1e) and grain size (Figure 1f). The pore density steadily decreased as the substrate temperature increased up to ∼470 °C and saturated beyond. In contrast to the pore density, the grain size increased steadily until ∼500 °C and remained stable beyond this temperature regime. However, it should be noted that at temperature greater than ∼550 °C there is a striking increase in the overall surface roughness (Supporting Information Figure S3). To investigate the mechanism underlying pore formation further, we examined a freshly cleaved mica surface before metal deposition, where AFM analysis revealed crystallite structures with varying dimensions on the mica surface (Figure
strated the complete removal of defects on the Au(111) surface by replacing mica with halide substrates, circumventing the need for complex multistep methodologies. Finally, we verify the formation of ordered, close-packed, pinhole-free C60 molecular layers on these ultraflat, ultrathin Au(111) textured surfaces reflecting their finer structural quality. Au layer of 100 nm was electron-beam evaporated onto commercially available mica and CaF2(111) substrates. (See Supporting Information for materials and deposition parameters.) The surface morphology of gold was checked with an atomic force microscope (AFM) in tapping mode (Bruker Dimension V, Hybrid scanner, “Supersharp” tips with
