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Nanoscale Origin of Defects at Metal/Molecule Engineered Interfaces
Abstract
Control and repair of defects at metal/molecule interfaces is central for 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.
... The physical and chemical properties are optimized to achieve the most suitable substrate for the study. In the case of metallic surfaces, such as gold, these optimizations have been achieved by studying the deposition parameters [1][2][3][4][5][6][7][8][9], the nature of the substrate [10][11][12][13][14], annealing [4,9,[11][12][13][14][15][16][17][18] and other surface treatments [5,11,13,[15][16][17]19,20]. One of the objectives is to increase the flatness of the surface. ...
... The physical and chemical properties are optimized to achieve the most suitable substrate for the study. In the case of metallic surfaces, such as gold, these optimizations have been achieved by studying the deposition parameters [1][2][3][4][5][6][7][8][9], the nature of the substrate [10][11][12][13][14], annealing [4,9,[11][12][13][14][15][16][17][18] and other surface treatments [5,11,13,[15][16][17]19,20]. One of the objectives is to increase the flatness of the surface. ...
... The physical and chemical properties are optimized to achieve the most suitable substrate for the study. In the case of metallic surfaces, such as gold, these optimizations have been achieved by studying the deposition parameters [1][2][3][4][5][6][7][8][9], the nature of the substrate [10][11][12][13][14], annealing [4,9,[11][12][13][14][15][16][17][18] and other surface treatments [5,11,13,[15][16][17]19,20]. One of the objectives is to increase the flatness of the surface. ...
Literature is well supplied with studies of gold deposit on mica for fabricating ultra-flat template-stripped gold surface. Gold on silicon has not received much attention. Our study optimizes the deposition rates, annealing temperatures and time to obtain a template-stripped gold surface with a very low roughness and large (111) oriented crystallites. Atomic force microscopy is performed in tapping and peak force mode to evaluate the surface roughness. The crystallite size is obtained from in situ X-ray diffraction measurements during annealing. We find out that a deposition rate of 0.2 nm/s with a 40 min annealing at 400°C gives the best result (0.21±0.04) nm.
... Note that, to date, the vast majority of AFM studies that focused on resolving the morphology and aggregation kinetics of A peptides have been conducted either on highly charged mica (29,30) or on chemically functionalized (21,25,26) mica. Other studies on freshly cleaved mica have shown that cleavage under ambient conditions can result in the formation of potassium carbonate crystals on the mica surface (31,32). Furthermore, when mica is exposed to buffer aqueous solution such as phosphate-buffered saline (PBS) (pH 7.2) and saline sodium citrate (pH 3.1), nanometer-sized buffer molecules coat the surface (33). ...
... We hypothesized that using atomically clean single-layer epitaxially grown graphene as an imaging platform could overcome the limitation of commonly used surfaces for protein imaging such as graphite that can structurally denature proteins (35) and mica that tends to attract ambient contaminants (31,32). To test this hypothesis, we deposited A-40 and A-42 peptides aggregated in PBS buffer aqueous solution (pH 7.4) onto a single-layer graphene surface (atomic structure and thickness verified through scanning tunneling microscopy; fig. ...
To visualize amyloid β (Aβ) aggregates requires an uncontaminated and artifact-free interface. This paper demonstrates the interface between graphene and pure water (verified to be atomically clean using tunneling microscopy) as an ideal platform for resolving size, shape, and morphology (measured by atomic force microscopy) of Aβ-40 and Aβ-42 peptide assemblies from 0.5 to 150 hours at a 5-hour time interval with single-particle resolution. After confirming faster aggregation of Aβ-42 in comparison to Aβ-40, a stable set of oligomers with a diameter distribution of ~7 to 9 nm was prevalently observed uniquely for Aβ-42 even after fibril appearance. The interaction energies between a distinct class of amyloid aggregates (dodecamers) and graphene was then quantified using molecular dynamics simulations. Last, differences in Aβ-40 and Aβ-42 networks were resolved, wherein only Aβ-42 fibrils were aligned through lateral interactions over micrometer-scale lengths, a property that could be exploited in the design of biofunctional materials.
... Experimental techniques that enable the measurement of single-molecule properties have signicantly advanced our understanding of how molecular structure relates to function, from charge transport 1,2 and magnetism [3][4][5] to mechanics. 6,7 In part, this achievement results from the decoupling of properties and variables associated with multi-molecular characterizations, 8,9 such as molecular packing effects, 10,11 defect types and densities, [12][13][14] or the absolute number of molecules being examined. 15,16 Though the primary focus has so far been characterization, there is an increasing need to address the problem of how useful properties might ultimately be exploited at the single-molecule level, for example, using chemical principles. ...
Whilst most studies in single-molecule electronics involve components first synthesized ex situ, there is also great potential in exploiting chemical transformations to prepare devices in situ. Here, as a first step towards this goal, we conduct reversible reactions on monolayers to make and break covalent bonds between alkanes of different lengths, then measure the conductance of these molecules connected between electrodes using the scanning tunneling microscopy-based break junction (STM-BJ) method. In doing so, we develop the critical methodology required for assembling and disassembling surface-bound single-molecule circuits. We identify effective reaction conditions for surface-bound reagents, and importantly demonstrate that the electronic characteristics of wires created in situ agrees with those created ex situ. Finally, we show that the STM-BJ technique is unique in its ability to definitively probe surface reaction yields both on a local (~50 nm²) and pseudo-global (≥10 mm²) level. This investigation thus highlights a route to the construction and integration of more complex, and ultimately functional, surface-based single-molecule circuitry, as well as advancing a methodology that facilitates studies beyond the reach of traditional ex situ synthetic approaches.
... Our data and analysis complements recent STM studies of molecule-induced rearrangements and adatom migration on gold, 14,28−33 which investigate the effect of surface features including grain boundaries and pores. 20, 34 We calculate a barrier of 3 eV for complete gold atom unbinding, consistent with the upper bound of literature values that range from approximately 1 to 3 eV, 35−37 depending on the method used to estimate binding strengths. By contrast, for gold atom migration via exchange between partially bound arrangements on step edges, termed rolling or "hopping", 4 we compute sub-1 eV barriers, similar to the sub-1 eV hopping barriers estimated from microscopy experiments, 38,39 and significantly lower than the barrier to complete unbinding. ...
Activation energies, Ea, measured from molecular exchange experiments are combined with atomic-scale calculations to describe the migration of bare Au atoms and Au-alkanethiolate species on gold nanoparticle surfaces during ligand exchange for the creation of metal-molecule-metal junctions. It is well-known that Au atoms and alkanethiol-Au species can diffuse on gold with sub-1 eV barriers, and surface restructuring is crucial for self-assembled monolayer (SAM) formation for interlinking nanoparticles and in contacting nanoparticles to electrodes. In the present work, computer simulations reveal that naturally occurring ridges and adlayers on Au(111) are etched and resculpted by migration of alkanethiolate-Au species toward high coordination kink sites at surface step edges. The calculated energy barrier, Eb, for diffusion via step edges is 0.4-0.7 eV, close to the experimentally measured Ea of 0.5-0.7 eV. By contrast, putative migration from isolated nine-coordinated terrace sites and complete Au unbinding from the surface incur significantly larger barriers of +1 and +3 eV, respectively. Molecular van der Waals packing energies are calculated to have negligible effect on migration barriers for typically used molecules (length < 2.5 nm), indicating that migration inside SAMs does not change the size of the migration barrier. We use the computational methodology to propose a means of creating Au nanoparticle arrays via selective replacement of citrate protector molecules by thiocyanate linker molecules on surface step sites. This work also outlines the possibility of using Au/Pt alloys as possible candidates for creation of contacts that are well-formed and long-lived because of the superior stability of Pt interfaces against atomic migration.
Alzheimer’s disease (AD) associated proteins exist in cerebrospinal fluid (CSF). This paper evidences that protein aggregate morphology distinctly differs in CSF of patients with AD dementia (ADD), mild cognitive impairment due to AD (MCI AD), with subjective cognitive decline without amyloid pathology (SCD) and with non-AD MCI using liquid-based atomic force microscopy (AFM). Spherical-shaped particles and nodular-shaped protofibrils were present in the CSF of SCD patients, whereas CSF of ADD patients abundantly contained elongated mature fibrils. Quantitative analysis of AFM topographs confirms fibril length is higher in CSF of ADD than in MCI AD and lowest in SCD and non-AD dementia patients. CSF fibril length is inversely correlated with CSF amyloid beta (Aβ) 42/40 ratio and CSF p-tau protein levels (obtained from biochemical assays) to predict amyloid and tau pathology with an accuracy of 94% and 82%, respectively, thus identifying ultralong protein fibrils in CSF as a possible signature of AD pathology.
Research on motion of molecules in the presence of thermal noise is central for progress in two-terminal molecular scale electronic devices. However, it is still unclear what influence imperfections in bottom metal electrode surface can have on molecular motion. Here, we report a two-layer crowding study, detailing the early-stages of surface motion of fullerene molecules on Au(111) with nanoscale pores in a n-tetradecane chemical environment. The motion of the fullerenes is directed by crowding of the underlying n-tetradecane molecules around the pore fringes at the liquid-solid interface. We observe in real-space the growth of molecular populations around different pore geometries. Supported by atomic-scale modelling, our findings extend the established picture of molecular crowding by revealing that trapped solvent molecules serve as prime nucleation sites at nanopore fringes.
Predicting the electronic framework of an organic molecule under practical conditions is essential if the molecules are to be wired in a realistic circuit. This demands a clear description of the molecular energy levels and dynamics as it adapts to the feedback from its evolving chemical environment and the surface topology. Here, we address this issue by monitoring in real-time the structural stability and intrinsic molecular resonance states of fullerene (C60)-based hybrid molecules in the presence of the solvent. Energetic levels of C60 hybrids are resolved by in situ scanning tunnelling spectroscopy with an energy resolution in the order of 0.1 eV at room-temperature. An ultra-thin organic spacer layer serves to limit contact metal-molecule energy overlap. The measured molecular conductance gap spread is statistically benchmarked against first principles electronic structure calculations and used to quantify the diversity in electronic species within a standard population of molecules. These findings provide important progress towards understanding conduction mechanisms at a single-molecular level and in serving as useful guidelines for rational design of robust nanoscale devices based on functional organic molecules.
Most fabrication methods for obtaining metal films rely on direct deposition techniques and usually yield surfaces with small grains and a significant surface roughness. In contrast, methods based on physical vapor deposition followed by template-stripping (TS) yield surfaces with large grains separated by smaller ones. We report a fabrication method that combines TS with annealing prior to TS. By optimizing the deposition rate, deposition temperature, and the annealing temperature and time, before TS, we were able to produce high quality Ag surfaces with low root-mean-square surface roughness (0.5 ± 0.1 nm), large grains of 0.84 ± 0.23 μm2, and a low surface fraction of pinholes of 0.01%. Thus, post-deposition annealing (prior to TS) changes the topography of AgTS surfaces significantly. The X-ray diffraction and X-ray photoelectron spectra show that the annealed AgTS surfaces are polycrystalline and consist of face-centered cubic Ag, do not contain silver oxides, and can be stored for several months. Ellipsometry shows that these AgTS surfaces have good optical properties and are promising for applications in plasmonics.
The drive towards organic computing is gaining momentum. Interestingly, the building blocks for such architectures is based on molecular ensembles extending from nucleic acids to synthetic molecules. Advancement in this direction requires devising precise nanoscopic experiments and model calculations to decipher the mechanisms governing the integration of a large number of molecules over time at room-temperature. Here, we report on ultrahigh-resolution scanning tunnelling microscopic measurements to register the motion of molecules in the absence of external stimulus in liquid medium. We observe the collective behavior of individual molecules within a swarm which constantly iterate their position to attain an energetically favourable site. Our approach provides a consistent pathway to register molecular self-assembly in sequential steps from visualising thermodynamically driven repair of defects up until the formation of a stable two-dimensional configuration. These elemental findings on molecular surface dynamics, self-repair and intermolecular kinetic pathways rationalised by atom-scale simulations can be explored for developing new models in algorithmic self-assembly to realisation of evolvable hardware.
Evaluating the built-in functionality of nanomaterials under practical conditions is central for their proposed integration as active components in next-generation electronics. Low-dimensional materials from single atoms to molecules have been consistently resolved and manipulated under ultrahigh vacuum at low temperatures. At room temperature, atomic-scale imaging has also been performed by probing materials at the solid/liquid interface. We exploit this electrical interface to develop a robust electronic decoupling platform that provides precise information on molecular energy levels recorded using in situ scanning tunnelling microscopy/spectroscopy with high spatial and energy resolution in a high-density liquid environment. Our experimental findings, supported by ab initio electronic structure calculations and atomic-scale molecular dynamics simulations, reveal direct mapping of single-molecule structure and resonance states at the solid/liquid interface. We further extend this approach to resolve
The implementation of optical tweezers at a surface opens a huge potential towards the elaboration of future lab-on-a-chip devices entirely operated with light. The transition from conventional three-dimensional (3D) tweezers to 2D is made possible by exploiting evanescent fields bound at interfaces. In particular, surface plasmons (SP) at metal/dielectric interfaces are expected to be excellent candidates to relax the requirements on incident power and to achieve subwavelength trapping volumes. Here, we report on novel 2D SP-based optical tweezers formed by finite gold areas fabricated at a glass surface, We demonstrate that SP enable stable trapping of single, dielectric beads under unfocused illumination with considerably reduced laser intensity compared with conventional optical tweezers. We show that the method can be extended to parallel trapping over any predefined pattern. Finally, we demonstrate how SP tweezers can be designed to selectively trap one type of particles out of a mixture, acting as an efficient optical sieve.
This paper describes the growth and characterization of large-scale ultra-smooth metal surfaces produced by an adapted replica
technique. Making use of this method, either amorphous or crystalline masters of different materials with ultra-flat surfaces,
e.g. mica, glass or polymer coatings on silicon, were coated by a physical vapor deposition (PVD) process with a thin precious-metal
layer. On the top of this layer a thick Ni surface was grown by electroplating. Both the precious-metal layer and the nickel
reinforcement can be stripped off from the master, and the free metal surface that is made to appear can be used as a substrate
for the self-assembly of molecules, mostly via chemisorption of thiol-functionalized moieties. The use of either gold or silver
layers led to films exhibiting different morphologies and roughnesses, which are all strongly influenced by the structure
of the master’s surface and by the conditions during the PVD-coating procedure. Utilizing mica as a master it was possible
to grow Ag and Au surfaces made of ultra-smooth well-defined <111>-oriented crystals. A root mean square roughness down to
0.2nm was measured over micrometer-sized areas by scanning tunneling microscopy. Very flat Au and Ag films have been also
produced using the amorphous masters.
Extremely thin gold layers were sputter deposited on glass and silicon substrates, and their thickness and morphology were studied by Rutherford backscattering (RBS) and atomic force microscopy (AFM) methods. The deposited layers change from discontinuous to continuous ones for longer deposition times. While the deposition rate on the silicon substrate is constant, nearly independent on the layer thickness, the rate on the glass substrate increases with increasing layer thickness. The observed dependence can be explained by a simple kinetic model, taking into account different sticking probabilities of gold atoms on a bare glass substrate and regions with gold coverage. Detailed analysis of the shape of the RBS gold signal shows that in the initial stages of the deposition, the gold layers on the glass substrate consist of gold islands with significantly different thicknesses. These findings were confirmed by AFM measurements, too. Gold coverage of the silicon substrate is rather homogeneous, consisting of tiny gold grains, but a pronounced worm-like structure is formed for the layer thickness at electrical continuity threshold. On the glass substrate, the gold clusters of different sizes are clearly observed. For later deposition stages, a clear tendency of the gold atoms to aggregate into larger clusters of approximately the same size is observed. At later deposition stages, gold clusters of up to 100 nm in diameter are formed.
A simple method to prepare flat gold terraces on mica for atomic force microscopy biomolecular characterisation is described. The procedure includes preheating of the substrate, metal deposition and an annealing step. All of these steps are at elevated temperatures (300-420degC). This approach allows one to prepare large flat gold terraces (200-500-nm), which constitute ideal substrates for visualisation and characterisation of a self-assembly monolayer of biomolecules at the nanoscale. The authors illustrated this potential of characterisation with the reconstitution of a protein monolayer.
Epitaxial alignment has been obtained by means of grain growth in polycrystalline films deposited on single‐crystal substrates. A theory for epitaxial grain growth is outlined and results given for experiments on Au, Al, Cu, and Ag films on vacuum‐cleaved NaCl, KBr, KCl, or mica. Epitaxial grain growth provides a fundamentally different alternative to conventional epitaxy, and can lead to very thin films with improved continuity and crystalline perfection, as well as non‐lattice‐matched orientations.
Silver, gold, palladium, and chromium films have been evaporated onto mica at substrate temperatures ranging from -150 °C to +400 °C. Scanning tunneling microscopy (STM) images were obtained under ambient conditions. Silver and gold films exhibit an increasing grain size together with a grain flattening as the substrate temperature increases. At 275 and 400 °C for silver and gold, respectively, terraces with dimensions of the order of more than 100 nm are formed. However, also characteristic holes on the scale of a few 10 nm were always present. In addition, the gold films exhibit characteristic holes on the nanometer scale. While the larger holes are also visible on scanning electron microscopy micrographs, the smaller are not. Low‐energy electron diffraction patterns prove the (111) orientation of both, silver and gold films; however, the quality of the silver patterns is better, consistent with the more perfect terraces as seen by STM. Palladium, evaporated at temperatures up to 350 °C did not exhibit similarly large and flat terraces. An improvement is achieved if prior to the palladium, silver is evaporated at elevated temperatures. Chromium, contrary to the fcc metals Ag, Au, and Pd crystallizes bcc, and forms a stable surface oxide under ambient conditions. The reproducibility of its STM images under ambient conditions depends on the applied bias and can be attributed to the surface oxide. Films evaporated at 50 °C exhibit a grainy structure. For the 350 °C films areas with a columnar morphology have been observed. In conclusion, epitaxially grown Ag(111) films evaporated at 275 °C onto mica have been shown to exhibit large flat terraces suitable for nanoscale modifications and chemisorption studies with the STM.
We have devised a method to produce gold films with ultralarge atomically flat areas for use in scanning probe microscopy. A means roughness of 2 Å for areas of 2.25 μm2, and about 3 Å for 25 μm2, can be easily produced. The method is based on: (i) epitaxial growth of gold on mica; gold is thermally deposited onto freshly cleaved mica sheets, (ii) glueing the fresh gold surface to a piece of Si wafer, and (iii) chemical or consecutive mechanical stripping of the mica down to the freshly appearing gold surface (i.e., template-stripped gold). We present a detailed STM and AFM roughness study of these gold films.
Many chemical reactions are catalysed by metal complexes, and insight into their mechanisms is essential for the design of future catalysts. A variety of conventional spectroscopic techniques are available for the study of reaction mechanisms at the ensemble level, and, only recently, fluorescence microscopy techniques have been applied to monitor single chemical reactions carried out on crystal faces and by enzymes. With scanning tunnelling microscopy (STM) it has become possible to obtain, during chemical reactions, spatial information at the atomic level. The majority of these STM studies have been carried out under ultrahigh vacuum, far removed from conditions encountered in laboratory processes. Here we report the single-molecule imaging of oxidation catalysis by monitoring, with STM, individual manganese porphyrin catalysts, in real time, at a liquid-solid interface. It is found that the oxygen atoms from an O2 molecule are bound to adjacent porphyrin catalysts on the surface before their incorporation into an alkene substrate.
The semiconductor industry has seen a remarkable miniaturization trend, driven by many scientific and technological innovations. But if this trend is to continue, and provide ever faster and cheaper computers, the size of microelectronic circuit components will soon need to reach the scale of atoms or molecules--a goal that will require conceptually new device structures. The idea that a few molecules, or even a single molecule, could be embedded between electrodes and perform the basic functions of digital electronics--rectification, amplification and storage--was first put forward in the mid-1970s. The concept is now realized for individual components, but the economic fabrication of complete circuits at the molecular level remains challenging because of the difficulty of connecting molecules to one another. A possible solution to this problem is 'mono-molecular' electronics, in which a single molecule will integrate the elementary functions and interconnections required for computation.
A technology is demonstrated to fabricate reliable metal-molecule-metal junctions with unprecedented device diameters up to 100 μm. The yield of these molecular junctions is close to unity. Preliminary stability investigations have shown a shelf life of years and no deterioration upon cycling. Key ingredients are the use of a conducting polymer layer (PEDOT:PSS) sandwiched between a bottom electrode with a self-assembled monolayer (SAM) and the top electrode to prevent electrical shorts, and processing in lithographically defined vertical interconnects (vias) to prevent both parasitic currents and interaction between the environment and the SAM [1].
Modeling the current–voltage ( I–V ) characteristics of alkanedithiols with the Simmons model showed that the low dielectric constant of the molecules in the junction results in a strong image potential that should be included in the tunneling model. Including image force effects, the tunneling model consistently describes the current-voltage characteristics of the molecular junctions up to 1 V bias for different molecule lengths [2].
Furthermore, we demonstrate a dependence of the I–V characteristics on the monolayer quality. A too low concentration of long alkanedithiols leads to the formation of looped molecules, resulting in a 50-fold increase of the current through the SAM. To obtain an almost full standing-up phase of 1,14-tetradecanedithiol (C14) a 30 mM concentration is required, whereas a 0.3 mM concentration leads to a highly looped monolayer. The conduction through the full standing-up phase of C14 and C16 is in accordance with the exponential dependence on molecular length as obtained from shorter alkanedithiols [3].
Finally, a fully functional solid-state molecular electronic switch is manufactured by conventional processing techniques. The molecular switch is based on a monolayer of photochromic diarylethene molecular switches. The monolayer reversibly switches the conductance by more than one order of magnitude between the two conductance states via optical addressing. This reversible conductance switch operates as an electronic ON/OFF switch (or a reprogrammable data storage unit) that can be optically written and electronically read [4].
Highly crystalline thin films of gold were grown on scratch-free mica. Epitaxial growth was accomplished by heating a fleshly cleaved 8 mm×8 mm piece of mica to 380 °C for 12 h at 1.9×10-7 Torr, followed by gold deposition at 1 Å/s and 380 °C until 100 nm was deposited. After flame annealing, these gold films are observed to have grains that are on average 6300 Å in width with a peak to peak topography of a few atomic layers. Adhesion of gold to mica was accomplished by controlling the temperature and length of bakeout in addition to the temperature of deposition. Films prepared in this manner exhibited excellent adhesion when immersed in all solvents except water. The nearly universally [111] terminated gold films were characterized by scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). STM imaging showed that the gold surfaces exhibit the 22×√3 reconstruction. Flame annealing of films removes contaminants and increases the flat surface area by a factor of 25 relative to the unannealed films as was indicated by both STM and TEM. © 1998 American Vacuum Society.
The ,positional order ,of dodecanethiol, monolayers ,self-assembled on Au(111) is investigated with scanning-tunneling microscopy,using ultra-high tunneling resistances R t ï* Wehave,studied two kinds of monolayers ,prepared (a) by immersion ,of the ,Au substrate ,into a methanoic thiol solution (as-adsorbed films), and (b) by an additional ex-situ heat treatment (annealed films). The head-group of the molecules are found to chemisorb in the commensurate ,3 3x overlayer of the ,Au(111) surface. As-adsorbed monolayers ,can be characterized by ,an assembly ,of nearest-neighbor (nn) rows of molecules with many missing nn-rows. Annealed films, on the other hand, are characterized by a 2x4 superstructure and zig-zag shaped rows of missing chemisorbed molecules. The two distinct structures are proposed ,to originate ,from two different lattices of the carbon-backbone orientational degree of freedom. Annealed films show, in addition, a considerable reduction of defect structures.
It is shown that the Pd(111) and Au(111) surfaces with wide atomically flat terraces having widths in a range of 2–10μm can be prepared by annealing in an argon stream. These surfaces were investigated by a laser confocal microscope combined with a differential interference contrast microscope (LCM–DIM). It is remarkable that regularly aligned step lines are clearly discerned in electrolyte solutions. The step lines observed by LCM–DIM are monatomic steps confirmed by scanning tunneling microscopy and atomic force microscopy. It is expected that LCM–DIM is a new powerful in situ method for the investigation of electrochemical reactions with the capability of atomic layer resolution.
High-quality atomically flat substrates are critical for the analysis and imaging of surface-mounted ultrathin films and nanostructures. Here we report significant improvement in the preparation of large areas of atomically smooth Au(111) substrates. A thin layer of gold on silicon is flame-annealed in air and then stripped from the template. The substrates were analyzed with X-ray diffraction and high-resolution atomic force microscopy (AFM). In contrast to the previously reported template stripped gold (TSG) substrates, flame-annealed template stripped substrates reveal no detectable pinholes. The substrate surface is atomically smooth with most grains being larger than 1 µm2. The entire procedure requires less than 2 h and uses readily available materials and common laboratory equipment. The resulting substrates can be stored for longer periods of time and then used immediately without need for common cleaning procedures. Evidence is provided that self-assembled monolayers on these substrates are higher quality than those prepared with previously reported gold substrates. Copyright © 2008 John Wiley & Sons, Ltd.
Self-assembled monolayers of alkanethiols (CnH2n+1SH; n = 3, 8, 12, 18, and 22) adsorbed on gold(111) are investigated with (atomically resolved) scanning tunneling microscopy (STM) and wetting measurements. The characteristic depressions observed in these monolayers with STM are proven to be holes in the underlying top gold surface layer rather than defects in the thiol monolayer itself. The holes originate from an etching process of the gold during adsorption of the thiol molecules: a correlation is obtained between the number of holes observed with STM and the amount of gold measured with atomic absorption spectroscopy in the thiol solution after adsorption. The erosion process is found to vanish as soon as complete self-assembly is observed in STM and wetting. For a dodecanethiol monolayer on gold adsorbed from a diluted methanoic thiol solution, self-assembly is observed within 10-min adsorption time. The average amount of gold in the thiol solution after 10 min corresponds to dissolution of 2% of a monolayer Au(111). The erosion strongly increases when the dodecanethiol adsorbs from undiluted thiol. The amount of holes also increases with decreasing thiol chain length as a result of a lower degree of self-assembly. The surface gold atoms underneath the thiol layer are highly mobile, which is manifest in STM tip-induced reorganization of the thiol layer and in penetration of evaporated gold through the thiol layer. This mobility is believed to be crucial in the etching process. Due to the mobility of thiol molecules during the adsorption process prior to acquiring a complete self-assembled structure, gold dissolves, probably in the form of a gold thiolate complex.
Localized voltage dependent capacitive currents through different self-assembled monolayers on Au(111) surfaces were studied by a spectroscopic method in a modified atomic force microscope that detects surface potentials and ∂C/∂z data as functions of an applied modulated and biased voltage between tip and sample. On flame-annealed Au surfaces, the ∂C/∂z signal is completely voltage independent. The signal amplitudes on 2-amino-alkanethiol layers change with both ac and dc voltage. The ∂C/∂z(Udc) data obtained represent the energy-dependent density of states of the investigated surface layer. Moreover, the charges of ligand stabilized clusters and the dependence of the clusters’ electrical properties on their size were reproduced. The results are discussed in connection with schematic energy models and replacement circuits.
The structures of hexadecanethiol (HDT) self-assembled monolayers (SAM) on gold assembled for up to 11 days from ethanolic HDT solutions are reported. Gold|HDT films were periodically removed from 0.001, 0.1, 1, or 10 mM HDT solution and analyzed by Fourier transform infrared external reflection spectroscopy (FTIR-ERS). FTIR-ERS data provide evidence for an ongoing increase in SAM crystallinity and, in some cases, a decrease in average chain tilt angle with respect to the surface normal during this long assembly time. SAM crystallinity, judged by line shape and position, increased more rapidly for more concentrated HDT solutions but appears to converge on the same final value independent of HDT concentration in the range of 10-6 to 10-2 M. Computer-optimized fits to the ensemble of C−H stretching bands between 2800 and 3000 cm-1 made on 50 different SAMs which were prepared under a wide range of conditions return surprisingly consistent average tilt and twist angles. For Au on Cr-primed glass the absolute values of the average cant and twist angles were 20 ± 2° and 49 ± 3°, respectively, and for Au on HF-etched Si111 they were 22 ± 2° and 51 ± 1°, respectively. A clear trend in SAM cant angles, decreasing with increasing time, is observed for the glass|Cr|Au|HDT system. Two structural models were compared: one in which all chains are identical and one in which alternating alkanethiol chains have orthogonal backbone planes.
We present a tunneling microscope investigation of step motion on Au(111) surfaces grown epitaxially on green mica. At ambient temperature and pressure, step edges of different heights move at typical rates of 50 angstrom/min over the course of an hour. Not all such surfaces have moving steps; surfaces with very low step densities are stable over an hour. On surfaces with moving steps, different parts of the steps move at different rates, so the shape of individual steps changes dramatically. We argue that this effect is not merely a mechanical scratching of the surface by the tip. This motion is not spontaneous but seems to be induced by external forces such as the bias between tip and surface, because step motion stops if tip scanning is stopped. These observations also hold for gold in a variety of aqueous solutions. We believe that the unstable behavior of the Au surfaces is due to a contamination during the preparation process. Thus we also present a detailed film preparation procedure for the gold on mica system.
Self-assembling (SA) monolayers of the aromatic compounds thiophenol (TP), p-biphenyl mercaptan (BPM), and p-terphenyl mercaptan (TPM) were prepared on gold electrodes. Contact angle (CA) measurements and ellipsometry indicate that TP forms poorly defined layers, while BPM and TPM form monolayers with reproducible CAs and ellipsometric thicknesses. BPM and TPM films are substantially more stable than TP layers toward various electrochemical conditions and replacement by alkanethiols as shown by the combined CAs, ellipsometric thicknesses, and the electrochemical measurements of the corresponding monolayer-covered electrodes. The surface coverages of the three monolayer systems, determined from the electrochemical responses of the respective electrodes, show that the blocking efficiency increases with the number of phenyl rings in the molecule. Three electrochemical techniques, namely gold oxide formation/removal, cyclic voltammetry (CV), and ac impedance were used to determine the monolayer surface coverage. The ac impedance technique proves to be superior for surface coverage determination, mainly due to the milder conditions applied. Molecular mechanics calculations show that BPM and TPM might form (square-root 3 x square-root 3)R30-degrees overlayers on Au(111) with the molecules nearly perpendicular to the gold surface. Such overlayers would have favorable intermolecular interactions and retain the usual structure for sulfur on Au(111).
The nature of the ''holes'' in self-assembled thiol monolayers-found in previous scanning-tunneling microscopy (STM) studies-is elucidated using STM with unprecedented high tunneling resistances (up to 1 TOMEGA). This enables the nondestructive imaging of dodecanethiol monolayers on Au(111) with atomic-scale resolution. The molecules are found to order in square-root 3 x square-root 3 domains separated by different types of missing row structures. This molecular ordering is typical not only for terraces but also for regions within ''holes''. These regions are therefore neither openings (pinholes) nor regions of disorder in the monolayer. Instead, they are attributes of the Au surface, which most likely originate from an etching process during the adsorption of the molecules.
We have previously described the preparation of ultraflat Au(111) surfaces as substrates for scanning probe microscopy. We report here alternative ways to produce polycrystalline Au(111) thin films of similar, high quality (i.e., with mean roughness smaller than 5 Angstrom over 25 mu m(2)). All of them are based on the same principle, i.e., that of exposing the very first layer of gold atoms which had deposited onto mica. One alternative route leads to substrates which are transparent enough for optical microscopy. Two other routes make use of ceramic glues, providing substrates which can be handled with most organic solvents without disruption of the Au(111) layer. The substrates prepared by the latter procedures can thus be used to produce gold-directed self-assembled monolayers (SAMs) from nearly all omega-functionalized alkanethiols or dialkyl disulfides; they can also be used for the in. situ chemical modification of SAMs on gold. For this purpose, appropriate reaction chambers have been developed. As an example of the use of the new techniques described here, we report the preparation of a SAM of N-palmitoylcysteamine, either from ex situ synthesis of the precursor or from in. situ acylation of the amino head groups of a cysteamine SAM.
The structure and dynamics of partial monolayers of C60 on the Au(111) surface were observed by scanning tunneling microscopy (STM) under ambient conditions. At submonolayer coverages, C60 molecules group into islands, and while the molecules in the interior of islands are quite stable, the C60s at island edges are less so, and their motions can be observed in real time using STM. The motion of surface-adsorbed C60 is predominantly the result of thermal diffusion; the extent of perturbation by the STM tip is determined to be minimal, using a quantitative analysis of a time series of STM images. Further analysis shows that motion of single C60 molecules is relatively uncommon, and that most diffusion of C60 on the surface occurs through the correlated or cooperative motion of molecular clusters ranging in size from 2 to 8 C60 molecules. Cluster diffusion is explained by a proposed “anchoring” mechanism, in which the stability of each C60 varies according to its position and orientation relative to the surface and to neighboring molecules.
It is well known that thin films develop large intrinsic stress during their preparation. The intrinsic stress either originates from strained regions within the films (grain boundaries, dislocations, voids, impurities, etc.) or at the film/substrate (lattice mismatch, different thermal expansion, etc) and film/vacuum interfaces (surface stress, adsorption, etc.) or is due to dynamic processes (recrystallization, interdiffusion, etc). Since the magnitude of most of these stress contributions is directly related to film morphology, important structural information can be extracted from measurements of the intrinsic stress. This article presents a thorough discussion of today's understanding of the growth of thin films and reviews the related atomistic mechanisms responsible for intrinsic stress. On the basis of these ideas recent experimental results on the intrinsic stress of UHV deposited polycrystalline and epitaxial thin metal films are discussed. Depending on the respective growth mode of the films-Volmer-Weber, Stranski-Krastanov and Frank-Van der Merwe modes-characteristic stress behaviours are observed. In situ intrinsic stress measurements are therefore a promising new technique to gain additional insight into film growth.
We systematically studied the influence of the substrate on the shape,
mobility, and stability of deposited gold clusters. The Au_n clusters were
produced in a laser vaporization source and deposited with low kinetic energy
(∼0.4 eV/atom) on atomically flat substrates (graphite, mica, and gold and
silver films on mica) under UHV conditions. Their size distribution is probed
with time-of-flight mass spectrometry and ranges from dimers to several
hundreds of atoms. Scanning probe microscopy is used to characterize the
deposited clusters and the formation of islands by cluster aggregation. On
all substrates, Au_n islands can be clearly distinguished and the islands are
flattened despite the small impact energy. The shape and size of the island
configurations are strongly system dependent. Gold clusters deposited on
Au(111) and Ag(111) films grown on mica do not aggregate, but deform due
to strong cluster–substrate interactions. The clusters tend to grow epitaxially
on these surfaces. On graphite and on mica, deposited clusters do diffuse
and aggregate. On the graphite surface, large ramified islands are formed by
juxtaposition of small islands and trapping of the clusters at the step edges. On
the other hand, the diffusion of the clusters on mica results in a total coalescence
of the Au_n clusters into compact islands.
A simple route to flat, large-area, single-crystalline films for plasmonics is demonstrated by sputter deposition of silver onto mica substrates at elevated temperatures. The films exhibit improved dielectric properties and allow more precise patterning of high-quality nanostructures for plasmonic applications.
Epitaxial Au films have been deposited on mica substrates at high temperatures (≳500 °C) and characterized by scanning tunneling microscopy, atomic force microscopy, and scanning electron microscopy. A critical growth temperature between 540 and 580 °C has been identified. Above this critical temperature, discontinuous films of micron‐sized Au island are obtained. The islands have the appearance of microcrystals with interisland spacings ranging from 0.2 to 1.5 μm and atomically flat surfaces similar to continuous Au films. © 1995 American Vacuum Society
The interface between a solution of C60 molecules in tetradecane and a Au(1 1 1) surface has been studied by in situ scanning tunneling microscopy. For a diluted solution, C60 islands with very mobile boundaries separated by disordered areas are found. While some two-dimension disordered C60 clusters are observed, the large majority of islands presents a periodic hexagonal packing of C60 molecules. This packing corresponds to either a 2√3×2√3 R30 arrangement with respect to Au(1 1 1) lattice or an in-phase overlayer. These structures agree well with the results obtained on ultra high vacuum-deposited C60 overlayers on the same substrate [Surf. Sci. 279 (1992) 49].
A simple and reproducible method for the preparation of gold films on mica with large (typically 0.8–1.0 μm) atomically flat (1 1 1) terraces is described. The procedure involves thin gold film evaporation onto freshly cleaved mica substrates, followed by 1 min annealing at 650 °C under nitrogen. The annealed gold surfaces are compared to those of freshly evaporated gold films on mica using cyclic voltammetry and atomic force microscopy. Our results favorably compare to other published annealing techniques, with minimal equipment and time necessary to reproducibly obtain atomically flat gold terraces.
We present transmission electron microscope observations of the effects of stress in polycrystalline gold thin films. The films exhibit a very strong {111} fiber texture, and are loaded in biaxial tensile stress. Under these conditions, each grain exhibits the same Schmid factor, within a very small variance. Despite the identical loading conditions and resolved shear stresses from grain to grain, we observe that yield is isolated in only a few grains. Yield initiates only at specific triple junctions, and we identify the conditions required for a triple junction to be active in this manner. The required conditions for yield to occur at a triple junction include (1) an appropriate slip-plane inclination relative to the grain boundaries at the junction; and (2) at least one grain boundary of appropriate character, adjacent to the slip plane specified in item 1.
The surface morphologies of thin gold films thermally evaporated on glass, mica, CaF2 and Si substrates were investigated by scanning tunneling microscopy (STM) and compared to each other. The surface roughness of the gold films and average area of the grains observed was investigated as a function of the temperature and length of time of prebake of the substrates. The dependence of the gold surface roughness on substrate temperature is discussed. For substrates held at room temperature the ionic interaction between gold particles and substrate surface determines the size of the grains. Large flat areas of dimensions of the order of 200×200 nm2 are obtained for 80 nm thick films grown on glass heated at 300°C for 6 h and areas flat over a macroscopic distance greater than 500 nm are obtained on mica heated at 400°C for the same period of time. For CaF2 gold epitaxial growth starts to occur above 200°C. Epitaxial growth of fcc metals on alkaline halides is discussed.
Templating against atomically flat materials allows creation of smooth metallic surfaces. The process of adding the backing (superstrate) to the deposited metals has proven to be the most difficult part in producing reliable, large-area, solvent-resistant substrates and has been the subject of recent research. In this paper we describe a simple and inexpensive liquid glass template-stripping (lgTS) method for the fabrication of large area ultraflat gold surfaces. Using our lgTS method, ultraflat gold surfaces with normals aligned along the <111> crystal plane and with a root-mean-square roughness of 0.275 nm (over 1 μm(2)) were created. The surfaces are fabricated on silica-based substrates which are highly solvent resistant and electrically insulating using silicate precursor solution (commonly known as "liquid glass") and concomitant mild heat treatment. We demonstrate the capabilities of such ultraflat gold surfaces by imaging nanoscale objects on top and fabricating microelectrodes as an example application. Because of the simplicity and versatility of the fabrication process, lgTS will have wide-ranging application in imaging, catalysis, electrochemistry, and surface science.
Ultraflat metal surfaces are used in template stripping (TS), which is a method for obtaining a metal with an average surface roughness on the order of <1 nm. This is important for plasmonics, for the production of high-quality SAM surfaces, and for many other applications. Herein we show for the first time that TS indeed introduces a very high density of surface nanodefects (twinning and stacking faults), which can strongly hinder surface-induced properties such as SAM ordering and plasmonic phenomena, despite the seemingly overall ultrahigh flatness. We have used state of the art characterization techniques such as HRXRD, spherical-aberration-corrected HRTEM, and STM. We also demonstrate how these nanodefects can be completely eliminated.
Muscovite mica is an important mineral that has become a standard substrate, due to its easy cleavage along the {001} planes, revealing a very flat surface that is compatible with many biological materials. Here we study mica surfaces by dynamic atomic force microscopy (AFM) operated in the non-contact mode (NC-AFM) under ultra-high vacuum (UHV) conditions. Surfaces produced by cleaving in UHV cannot be imaged with NC-AFM due to large surface charges; however, cleavage in air yields much less surface charge and allows for NC-AFM imaging. We present highly resolved NC-AFM images of air-cleaved mica surfaces revealing a rough morphology originating from a high density of nanometre-sized particles. Among these particles, we find regularly shaped structures indicating the growth of crystallites on the surface. The contamination layer cannot be removed by degassing in UHV; even prolonged heating at a temperature of 560 K under UHV conditions does not yield an atomically flat surface.
A new technology for the fabrication of reliable solid-state molecular devices using a graphene multilayer as the top electrode is introduced. Graphene-electrode molecular devices were fabricated in high yield with good junction conductance. These devices also have excellent durabilities, thermal and operational stabilities, and device lifetimes.
Using scanning tunnelling microscopy (STM), we have studied mixed self-assembled monolayers of linear alkanethiol molecules. Nonanedithiol (C9S2), nonanethiol (C9S), decanethiol (C10S), and dodecanethiol (C12S) were inserted into a self-assembled octanethiol (C8S) host matrix monolayer on an Au(111) surface using a two-step method. Quasi-one-dimensional double-row structures were found in the ordered, close-packed domains of the C8S matrix for each mixed monolayer system. These close-packed domains coexist with ordered striped phase domains (for C9S and C10S) or with a disordered phase (for C9S2 and C12S). Results from high-resolution images suggest that the double-rows are composed of inserted non-nearest-neighbor substitutional molecules, the ordering of which may be a result of locally induced surface stress.
n-Alkanethiol (CnH2n+1SH) self-assembled monolayers (SAMs) adsorbed on Au(111) were studied with an atomic force microscope (AFM) to confirm the influence of the lateral interaction between adsorbed thiols on the film morphology. Two experiments were performed: firstly, a study of the domain formation at the initial stage of SAM growth (single component) and, secondly, investigations of the coadsorption phenomenon in mired SAMs composed of two alkanethiols having different chain lengths, For the kinetics study, Au(111) was immersed into the 10(-2) mM ethanol solutions with the single component alkanethiol (C4H9SH, C12H25SH, or C18H37SH), for varying times (1 s to 10 min). In all cases, the film coverage increased as the immersion time became longer, and finally the surface was totally covered with thiols after an immersion time of 3 min or more. Clear island formations were observed in the partially covered C12H25SH and C18H37SH SAMs, while C4H9SH formed meshlike domains. The mixed SAMs were prepared by immersing Au(111) into 1 mM ethanol solutions with mixed alkanethiols (C4H9SH/C18H37SH) of various compositions, R(soln) = [C4H9SH]/[C18H37SH] = 1/1 to 100/1, for a time of 1 h. Clear phase separation was observed at R(soln) = 20/1 and 40/1. Above or below these compositions, the film surface appeared very flat, covered with a nearly single component, C4H9SH or C18H37SH, respectively. This is the first systematic study of the surface phase behavior of alkanethiol SAMs by AFM imaging. It reveals more direct information about the film morphology than previous studies with conventional surface analytical techniques such as X-ray photoelectron spectroscopy, ellipsometry, contact angles, etc.
Air-cleaved mica surfaces exhibit a high density of nanometer or micrometer size particles that have been ascribed to potassium carbonate formed as a reaction product of carbonaceous gases with potassium ions. Unambiguous evidence for this assignment has, however, never been presented. We study air-cleaved mica surfaces by high-resolution noncontact atomic force microscopy (NC-AFM) in ultrahigh vacuum to reveal the detailed structure of such precipitates on the surface. Among a large number of irregularly shaped surface structures, we find flat, hexagonally shaped islands exhibiting two different patterns on their surfaces, namely a rectangular atomic corrugation pattern and a hexagonal moire structure. The unit cell of the rectangular pattern corresponds to the dimensions of the potassium carbonate bulk structure and is found on high crystallites. The moire structure solely appears on very flat islands and is caused by the interference of the potassium carbonate lattice periodicity and the lattice periodicity of the underlying mica substrate. Both results strongly point to the presence of potassium carbonate crystallites on air-cleaved mica surfaces.
Using the electrochemical surface forces apparatus, we investigated adhesion (from pull-off measurements) between gold and mica as the potential of the gold surface was changed externally. Measurements were performed at different concentrations of KClO(4) in a potential window where the gold electrode is ideally polarizable. At applied potentials where the gold-mica interactions are repulsive, we obtain double layer forces that are predictable by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloid stability but deviate from the theory at short range. At applied potentials where the gold-mica interactions are attractive, we observed a very strong dependence of adhesion on the applied potential, a result that cannot be directly related to DLVO theory. We show, however, that an approach based on electrocapillary thermodynamics can be employed to model the potential dependence of adhesion seen in our measurements. This electrocapillary approach presents evidence of charging at the gold-mica interface and stresses the relation between the charge within and outside of the contact area.
A simple procedure using gold diffusion bonding for the preparation of template-stripped gold (TSG) surfaces is described. TSG surfaces are useful for surface studies because a very consistent flat gold surface with few defects can be easily prepared. We have developed a method of producing TSG surfaces that relies only on gold diffusion bonding rather than epoxies. The resulting substrates are free from concerns of solvent compatibility, heat stability, and impurities. Bonding of centimeter-sized substrates is performed at 300 degrees C for 2 h using a vise and aluminum foil.
Evaporated gold films are frequently used as substrates for the study of biomolecular adsorbates, nanoparticle systems, amd partial and full monolayer films. These studies often benefit from a predeposition cleaning of the surface that removes adventitiously adsorbed material from laboratory contaminants. Scanning tunneling microscopy (STM) is used in this study to explore the microscopic consequences of two pretreatment protocols used in literature reports of self-assembled monolayers, based on sulfochromic and piranha acid solutions. These measurements show that treatment of the Au/mica surface with piranha acid can lead to extensive and uncontrolled etching of the surface and severe disruption of the surface topography; extended exposure causes the precipitation of crystallites on the surface that are highly mobile during STM imaging processes. Exposure of Au/mica surfaces to sulfochromic acid leads to the formation of permanent etch pits of the surface that are exclusively one Au layer deep; extended exposure leads to progressive etching and oxidation of the surface, ultimately leading to the formation of 0.33-0.36 nm high islands on the otherwise flat Au/mica surface. The piranha acid solutions are significantly more likely to cause the Au film to delaminate from the mica support than are the sulfochromic acid solutions. These results show that sulfochromic surface preparation is a direct and reliable method for the elimination of organic residues from Au(111)-textured surfaces, while causing a minimum of structural and chemical surface damage.
A simple procedure to elaborate robust ultraflat gold surface without clean room facilities is presented. Self-assembled 3-mercaptopropytriethoxysilane (MPTMS) on silicon was used as a buffer layer on which gold was sputtered using a common sputter-coating apparatus. The optimization of the sample position into the chamber of the sputtering machine yielded the formation of a thin (approximately 8 nm) gold layer. The characterization of the resulting gold surface (i.e., AFM, X-ray reflectivity, and diffraction) has demonstrated its high smoothness (<0.7 nm) over a large scale with a preferred (111) orientation. The robustness of the substrate toward organic solvents and thermal treatment was also tested. The ability of these surfaces to be used as substrates for high-resolution surface modification was confirmed by functionalizing the gold surface using the dip pen nanolithography process.
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