The use of focused ion beam (FIB) milling to fabricate nanochannels with critical dimensions extending below 5 nm is described. FIB milled lines have narrowing widths as they are milled deeper into a ...substrate. This focusing characteristic is coupled with a two-layered architecture consisting of a relatively thick (>100 nm) metal film deposited onto a substrate. A channel is milled through the metal layer until it penetrates a prescribed depth into the substrate material. The metal is then removed, leaving a nanochannel with smooth surfaces and lateral dimensions as small as sub-5 nm. These open nanochannels can be sealed with a cover plate and the resulting devices are well-suited for single-molecule DNA transport studies. This methodology is used with quartz, single-crystal silicon, and polydimethylsiloxane substrates to demonstrate its general utility.
The ability to precisely control the transport of single DNA molecules through a nanoscale channel is critical to DNA sequencing and mapping technologies that are currently under development. Here we ...show how the electrokinetically driven introduction of DNA molecules into a nanochannel is facilitated by incorporating a three-dimensional nanofunnel at the nanochannel entrance. Individual DNA molecules are imaged as they attempt to overcome the entropic barrier to nanochannel entry through nanofunnels with various shapes. Theoretical modeling of this behavior reveals the pushing and pulling forces that result in up to a 30-fold reduction in the threshold electric field needed to initiate nanochannel entry. In some cases, DNA molecules are stably trapped and axially positioned within a nanofunnel at sub-threshold electric field strengths, suggesting the utility of nanofunnels as force spectroscopy tools. These applications illustrate the benefit of finely tuning nanoscale conduit geometries, which can be designed using the theoretical model developed here.Forcing a DNA molecule into a nanoscale channel requires overcoming the free energy barrier associated with confinement. Here, the authors show that DNA injected through a funnel-shaped entrance more efficiently enters the nanochannel, thanks to facilitating forces generated by the nanofunnel geometry.
The electrophoretically driven transport of double-stranded λ-phage DNA through focused ion beam (FIB) milled nanochannels is described. Nanochannels were fabricated having critical dimensions (width ...and depth) corresponding to 0.5×, 1×, and 2× the DNA persistence length, or 25 nm, 50 nm, and 100 nm, respectively. The threshold field strength required to drive transport, the threading mobility, and the transport mobility were measured as a function of nanochannel size. As the nanochannel dimensions decreased, the entropic barrier to translocation increased and transport became more constrained. Equilibrium models of confinement provide a framework in which to understand the observed trends, although the dynamic nature of the experiments resulted in significant deviations from theory. It was also demonstrated that the use of dynamic wall coatings for the purpose of electroosmotic flow suppression can have a significant impact on transport dynamics that may obfuscate entropic contributions. The nonintermittent DNA transport through the FIB milled nanochannels demonstrates that they are well suited for use in nanofluidic devices. We expect that an understanding of the dynamic transport properties reported here will facilitate the incorporation of FIB-milled nanochannels in devices for single molecule and ensemble analyses.
Experiments measuring the extension of moderately confined double-stranded DNA within nanochannels have consistently shown a stronger scaling relationship between extension length and nanochannel ...confining dimensions than predicted by theory. Contrary to past findings, in this article the DNA extension length (R) was found to scale with D eff in close agreement with the theoretically predicted relationship (R ∼ D eff –2/3) and simulation results, with best-fit exponent values ranging from −0.67 to −0.70 across three unique devices. In addition, the power law fits of the experimental data exhibited close agreement with an exact model in the extended de Gennes regime, with fit prefactors within 10% of the expected value. A comparison of device dimensions against those used in previous experiments is presented to reconcile current findings with past results, suggesting that modest aspect ratios in rectilinear channels do not appreciably affect scaling, whereas the smallest dimension of a nanochannel can strongly impact extension. The results are also in quantitative agreement with recent unified theories describing the scaling of DNA extension across various conformational regimes.
The structural dynamics−cluster size and adsorbate-dependent thermal behaviors of the metal−metal (M−M) bond distances and interatomic order−of Pt nanoclusters supported on a γ-Al2O3 are described. ...Data from scanning transmission electron microscopy (STEM) and X-ray absorption spectroscopy (XAS) studies reveal that these materials possess a dramatically nonbulklike nature. Under an inert atmosphere small, subnanometer Pt/γ-Al2O3 clusters exhibit marked relaxations of the M−M bond distances, negative thermal expansion (NTE) with an average linear thermal expansion coefficient α = (−2.4 ± 0.4) × 10−5 K−1, large static disorder and dynamical bond (interatomic) disorder that is poorly modeled within the constraints of classical theory. The data further demonstrate a significant temperature-dependence to the electronic structure of the Pt clusters, thereby suggesting the necessity of an active model to describe the cluster/support interactions mediating the cluster’s dynamical structure. The quantitative dependences of these nonbulklike behaviors on cluster size (0.9 to 2.9 nm), ambient atmosphere (He, 4% H2 in He or 20% O2 in He) and support identity (γ-Al2O3 or carbon black) are systematically investigated. We show that the nonbulk structural, electronic and dynamical perturbations are most dramatically evidenced for the smallest clusters. The adsorption of hydrogen on the clusters leads to an increase of the Pt−Pt bondlengths (due to a lifting of the surface relaxation) and significant attenuation of the disorder present in the system. Oxidation of these same clusters has the opposite effect, leading to an increase in Pt−Pt bond strain and subsequent enhancement in nonbulklike thermal properties. The structural and electronic properties of Pt nanoclusters supported on carbon black contrast markedly with those of the Pt/γ-Al2O3 samples in that neither NTE nor comparable levels of atomic disorder are observed. The Pt/C nanoclusters do exhibit, however, both size- and adsorbate-induced trends in bond strain that are similar to those of their Pt/γ-Al2O3 analogues. Taken together, the data highlight the significant role that electronic effects − specifically charge exchange due to both metal−support and metal−adsorbate interactions − play in mediating the structural dynamics of supported nanoscale metal clusters that are broadly used as heterogeneous catalysts.
A nanofluidic device is described that is capable of electrically monitoring the driven translocation of DNA molecules through a nanochannel. This is achieved by intersecting a long transport channel ...with a shorter orthogonal nanochannel. The ionic conductance of this transverse nanochannel is monitored while DNA is electrokinetically driven through the transport channel. When DNA passes the intersection, the transverse conductance is altered, resulting in a transient current response. In 1 M KCl solutions, this was found to be a current enhancement of 5–25%, relative to the baseline transverse ionic current. Two different device geometries were investigated. In one device, the DNA was detected after it was fully inserted into and translocating through the transport nanochannel. In the other device, the DNA was detected while it was in the process of entering the nanochannel. It was found that these two conditions are characterized by different transport dynamics. Simultaneous optical and electrical monitoring of DNA translocation confirmed that the transient events originated from DNA transport through the nanochannel intersection.
Variable-temperature X-ray absorption spectroscopy measurements of sub-nanometer Pt nanoparticles on a high-surface-area γ-alumina support reveal that the Pt−Pt bonds exhibit contraction upon heating ...i.e., negative thermal expansion (NTE). Bare clusters under a He environment show an average linear expansion coefficient of −2.5 × 10-5 K-1 over the temperature range studied. Adsorption of hydrogen results in an overall bond relaxation and less dramatic Pt−Pt thermal bond-length contraction. From the temperature effect on bond length, disorder parameters, and the X-ray absorption near edge structure (XANES) spectra, temperature-dependent charge transfer between the support and the Pt clusters was concluded to be responsible for the observed behavior.
The structure of Au heterogeneous catalysts on the anatase form of TiO2 has been characterized by high-resolution electron microscopy (HREM) and quantitative Z-contrast scanning transmission electron ...microscopy (STEM). These materials are prepared by deposition of highly monodisperse Au13PPh34S(CH2)11CH34 (8-(symbol) diameter) ligand-protected clusters on anatase, followed by reaction with ozone or a rapid oxidative thermal treatment to remove the ligands. The materials obtained differ markedly in each case. For the thermal treatment at 400 degrees C in air, the supported particles grow to an average size of 2.7 nm (+/- 0.6 nm) in diameter, and the larger particles in the distribution are found to adopt a spherical geometry. Particle growth is greatly inhibited when ozone is used to remove the ligands (average diameter 1.2 +/- 0.5 nm). These particles assume a more oblate geometry, consistent with a truncated hemispherical shape. It was found that subsequent thermal treatments of the ozone-derived supported nanoparticles at 400 degrees C in air did not induce additional growth, indicating that sintering is strongly affected by the particle-support interactions developed by the ozone-based low-temperature ligand removal step. These materials exhibit catalytic activity and high stability for the oxidation of CO at elevated temperatures, with the level of activity dependent on catalyst preparation. PUBLICATION ABSTRACT
The atomic metal core structures of the subnanometer clusters Au13PPh34S(CH2)11CH32Cl2 (1) and Au13PPh34S(CH2)11CH34 (2) were characterized using advanced methods of electron microscopy and X-ray ...absorption spectroscopy. The number of gold atoms in the cores of these two clusters was determined quantitatively using high-angle annular dark field scanning transmission electron microscopy. Multiple-scattering-path analyses of extended X-ray absorption fine structure (EXAFS) spectra suggest that the Au metal cores of each of these complexes adopt an icosahedral structure with a relaxation of the icosahedral strain. Data from microscopy and spectroscopy studies extended to larger thiolate-protected gold clusters showing a broader distribution in nanoparticle core sizes (183 ± 116 Au atoms) reveal a bulklike fcc structure. These results further support a model for the monolayer-protected clusters (MPCs) in which the thiolate ligands bond preferentially at 3-fold atomic sites on the nanoparticle surface, establishing an average composition for the MPC of Au180S(CH2)11CH340. Results from EXAFS measurements of a gold(I) dodecanethiolate polymer are presented that offer an alternative explanation for observations in previous reports that were interpreted as indicating Au MPC structures consisting of a Au core, Au2S shell, and thiolate monolayer.
This study describes a prototypical, bimetallic heterogeneous catalyst: compositionally well-defined Ir−Pt nanoclusters with sizes in the range of 1−2 nm supported on γ-Al2O3. Deposition of the ...molecular bimetallic cluster Ir3Pt3(μ-CO)3(CO)3(η-C5Me5)3 on γ-Al2O3, and its subsequent reduction with hydrogen, provides highly dispersed supported bimetallic Ir−Pt nanoparticles. Using spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) and theoretical modeling of synchrotron-based X-ray absorption spectroscopy (XAS) measurements, our studies provide unambiguous structural assignments for this model catalytic system. The atomic resolution Cs-STEM images reveal strong and specific lattice-directed strains in the clusters that follow local bonding configurations of the γ-Al2O3 support. Combined nanobeam diffraction (NBD) and high-resolution transmission electron microscopy (HRTEM) data suggest the polycrystalline γ-Al2O3 support material predominantly exposes (001) and (011) surface planes (ones commensurate with the zone axis orientations frequently exhibited by the bimetallic clusters). The data reveal that the supported bimetallic clusters exhibit complex patterns of structural dynamics, ones evidencing perturbations of an underlying oblate/hemispherical cuboctahedral cluster−core geometry with cores that are enriched in Ir (a result consistent with models based on surface energetics, which favor an ambient cluster termination by Pt) due to the dynamical responses of the M−M bonding to the specifics of the adsorbate and metal−support interactions. Taken together, the data demonstrate that strong temperature-dependent charge-transfer effects occur that are likely mediated variably by the cluster−support, cluster−adsorbate, and intermetallic bonding interactions.
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