Study of radiative heat transfer in Angstrom- and nanometre-sized gaps.
ABSTRACT: Radiative heat transfer in Ångström- and nanometre-sized gaps is of great interest because of both its technological importance and open questions regarding the physics of energy transfer in this regime. Here we report studies of radiative heat transfer in few Å to 5?nm gap sizes, performed under ultrahigh vacuum conditions between a Au-coated probe featuring embedded nanoscale thermocouples and a heated planar Au substrate that were both subjected to various surface-cleaning procedures. By drawing on the apparent tunnelling barrier height as a signature of cleanliness, we found that upon systematically cleaning via a plasma or locally pushing the tip into the substrate by a few nanometres, the observed radiative conductances decreased from unexpectedly large values to extremely small ones-below the detection limit of our probe-as expected from our computational results. Our results show that it is possible to avoid the confounding effects of surface contamination and systematically study thermal radiation in Ångström- and nanometre-sized gaps.
Project description:Interferometric techniques have proven useful to infer proximity and local surface profiles of microscopic objects near surfaces. But a critical trade-off emerges between accuracy and mathematical complexity when these methods are applied outside the vicinity of closest approach. Here we introduce a significant advancement that enables reflection interference contrast microscopy to provide nearly instantaneous reconstruction of an arbitrary convex object's contour next to a bounding surface with nanometre resolution, making it possible to interrogate microparticle/surface interaction phenomena at radii of curvature 1,000 times smaller than those accessible by the conventional surface force apparatus. The unique view-from-below perspective of reflection interference contrast microscopy also reveals previously unseen deformations and allows the first direct observation of femtolitre-scale capillary condensation dynamics underneath micron-sized particles. Our implementation of reflection interference contrast microscopy provides a generally applicable nanometre-scale resolution tool that can be potentially exploited to dynamically probe ensembles of objects near surfaces so that statistical/probabilistic behaviour can be realistically captured.
Project description:Although deformation processes in submicron-sized metallic crystals are well documented, the direct observation of deformation mechanisms in crystals with dimensions below the sub-10-nm range is currently lacking. Here, through in situ high-resolution transmission electron microscopy (HRTEM) observations, we show that (1) in sharp contrast to what happens in bulk materials, in which plasticity is mediated by dislocation emission from Frank-Read sources and multiplication, partial dislocations emitted from free surfaces dominate the deformation of gold (Au) nanocrystals; (2) the crystallographic orientation (Schmid factor) is not the only factor in determining the deformation mechanism of nanometre-sized Au; and (3) the Au nanocrystal exhibits a phase transformation from a face-centered cubic to a body-centered tetragonal structure after failure. These findings provide direct experimental evidence for the vast amount of theoretical modelling on the deformation mechanisms of nanomaterials that have appeared in recent years.
Project description:DNA sequencing with a solid-state nanopore requires a reduction of the translocation speeds of single-stranded DNA (ssDNA) over 10??s/base. In this study, we report that a nanometre-sized bead structure constructed around a nanopore can reduce the moving speed of ssDNA to 270??s/base by adjusting the diameter of the bead and its surface chemical group. This decelerating effect originates from the strong interaction between ssDNA and the chemical group on the surface of the bead. This nanostructure was simply prepared by dip coating in which a substrate with a nanopore was immersed in a silica bead solution and then dried in an oven. As compared with conventional approaches, our novel method is less laborious, simpler to perform and more effective in reducing ssDNA translocation speed.
Project description:Au nanoparticles (NPs) deposited on a substrate function as ring shaped colloidal shadow masks. Using e-beam evaporation of gold, nanometer sized gaps are formed as a result. The size of these gaps can be accurately tuned by controlling the thickness of the gold deposition, thereby tuning the plasmonic coupling of the NPs with the substrate. The clean cavity produced between the Au NPs and the Au film provides an excellent SERS platform for trace molecule detection.
Project description:Meeting the nanometre resolution promised by super-resolution microscopy techniques (pointillist: PALM, STORM, scanning: STED) requires stabilizing the sample drifts in real time during the whole acquisition process. Metal nanoparticles are excellent probes to track the lateral drifts as they provide crisp and photostable information. However, achieving nanometre axial super-localization is still a major challenge, as diffraction imposes large depths-of-fields. Here we demonstrate fast full three-dimensional nanometre super-localization of gold nanoparticles through simultaneous intensity and phase imaging with a wavefront-sensing camera based on quadriwave lateral shearing interferometry. We show how to combine the intensity and phase information to provide the key to the third axial dimension. Presently, we demonstrate even in the occurrence of large three-dimensional fluctuations of several microns, unprecedented sub-nanometre localization accuracies down to 0.7?nm in lateral and 2.7?nm in axial directions at 50 frames per second. We demonstrate that nanoscale stabilization greatly enhances the image quality and resolution in direct stochastic optical reconstruction microscopy imaging.
Project description:Confocal Raman spectroscopy has emerged as a major, versatile workhorse for the non-invasive characterization of graphene. Although it is successfully used to determine the number of layers, the quality of edges, and the effects of strain, doping and disorder, the nature of the experimentally observed broadening of the most prominent Raman 2D line has remained unclear. Here we show that the observed 2D line width contains valuable information on strain variations in graphene on length scales far below the laser spot size, that is, on the nanometre-scale. This finding is highly relevant as it has been shown recently that such nanometre-scaled strain variations limit the carrier mobility in high-quality graphene devices. Consequently, the 2D line width is a good and easily accessible quantity for classifying the crystalline quality, nanometre-scale flatness as well as local electronic properties of graphene, all important for future scientific and industrial applications.
Project description:Super-Planckian near-field radiative heat transfer allows effective heat transfer between a hot and a cold body to increase beyond the limits long known for black bodies. Until present, experimental techniques to measure the radiative heat flow relied on steady-state systems. Here, we present a dynamic measurement approach based on the transient plane source technique, which extracts thermal properties from a temperature transient caused by a step input power function. Using this versatile method, that requires only single sided contact, we measure enhanced radiative conduction up to 16 times higher than the blackbody limit on centimeter sized glass samples without any specialized sample preparation or nanofabrication.
Project description:Radiative heat exchange at the nanoscale presents a challenge for several areas due to its scope and nature. Here, we provide a thermokinetic description of microscale radiative energy transfer including phonon-photon coupling manifested through a non-Debye relaxation behavior. We show that a lognormal-like distribution of modes of relaxation accounts for this non-Debye relaxation behavior leading to the thermal conductance. We also discuss the validity of the fluctuation-dissipation theorem. The general expression for the thermal conductance we obtain fits existing experimental results with remarkable accuracy. Accordingly, our approach offers an overall explanation of radiative energy transfer through micrometric gaps regardless of geometrical configurations and distances.
Project description:Binary polymer brush patterns were fabricated via photodeprotection of an aminosilane with a photo-cleavable nitrophenyl protecting group. UV exposure of the silane film through a mask yields micrometre-scale amine-terminated regions that can be derivatised to incorporate a bromine initiator to facilitate polymer brush growth via atom transfer radical polymerisation (ATRP). Atomic force microscopy (AFM) and imaging secondary ion mass spectrometry (SIMS) confirm that relatively thick brushes can be grown with high spatial confinement. Nanometre-scale patterns were formed by using a Lloyd's mirror interferometer to expose the nitrophenyl-protected aminosilane film. In exposed regions, protein-resistant poly(oligo(ethylene glycol)methyl ether methacrylate) (POEGMEMA) brushes were grown by ATRP and used to define channels as narrow as 141 nm into which proteins could be adsorbed. The contrast in the pattern can be inverted by (i) a simple blocking reaction after UV exposure, (ii) a second deprotection step to expose previously intact protecting groups, and (iii) subsequent brush growth via surface ATRP. Alternatively, two-component brush patterns can be formed. Exposure of a nitrophenyl-protected aminosilane layer either through a mask or to an interferogram, enables growth of an initial POEGMEMA brush. Subsequent UV exposure of the previously intact regions allows attachment of ATRP initiator sites and growth of a second poly(cysteine methacrylate) (PCysMA) brush within photolithographically-defined micrometre or nanometre scale regions. POEGMEMA brushes resist deposition of liposomes, but fluorescence recovery after photobleaching (FRAP) studies confirm that liposomes readily rupture on PCysMA "corrals" defined within POEGMEMA "walls". This leads to the formation of highly mobile supported lipid bilayers that exhibit similar diffusion coefficients to lipid bilayers formed on surfaces such as glass.
Project description:We determined the optimum dispersant to separate bulk Nb2Se9 material into 1D chain units. The Nb2Se9, which had a negative zeta potential (-43.3?mV), showed acidic characteristics and strongly bonded with the amine head of octadecyl amine through a charge transfer (from the amine to Se atoms) reaction. The steric hindrance of the octadecyl tail resulted in excellent dispersion of Nb2Se9 (down to nanometre-sized mono-chains).