Bio-imaging requires robust ultra-bright probes without causing any toxicity to the cellular environment, maintain their
stability and are chemically inert. In this work we present hexagonal boron nitride (hBN) nanoflakes which exhibit
narrowband ultra-bright single photon emitters1. The emitters are optically stable at room temperature and under ambient
environment. hBN has also been noted to be noncytotoxic and seen significant advances in functionalization with
biomolecules2,3. We further demonstrate two methods of engineering this new range of extremely robust multicolour
emitters across the visible and near infrared spectral ranges for large scale sensing and biolabeling applications.
Coatings that can self-modulate their optical properties as a function of an external stimulus are of significant technological interest. In this regard, the possibilities for thermo- or electrochromic materials such as VO2 and WO3 are already comparatively well-known. Here, however, we explore a new kind of 'smart' coating, based on the active control of a plasmon resonance in nanoparticles. One possible system is based on the modulation of the plasmon resonance of a precious metal nanorod or nanosphere by an active dielectric shell. The active dielectric undergoes an insulator-to-metal transition on increase of temperature which modulates the plasmon resonance of the underlying precious metal nanoparticle, thereby changing the wavelength at which its optical extinction is maximum. In the case of nanorods, the absorption maximum of the longitudinal plasmon is particularly sensitive to the aspect ratio of the nanoparticle and the dielectric properties of the environment, and may be readily tuned across the visible and near-infrared portions of the spectrum. In addition, nanoparticles of certain thermochromic dielectrics, such as VO2, are expected to have a plasmon resonance of their own which can be switched on or off by control of the temperature. We consider some of the possibilities, using both the discrete dipole approximation and the exact analytical solution due to Mie to calculate the optical properties.
We report on first principles calculations of the tunneling current across n-alkanedithiol molecules (n = 4,6,8,10,12) sandwiched between two Au {111} electrodes. The conductance drops exponentially with increased chain length with decay parameter βn = 0.9. The results are compared with scanning tunneling microscopy measurements on decanedithiol and with other n-alkanedithiol (n = 6,8,10) results in the literature. The theoretical results are found to be an order of magnitude larger than experimental values but follow the same trend. However, two additional, more realistic, geometries are modeled by changing the bond type and by combining the first-principles results with a Wentzel-Kramer-Brillouin (WKB) expression for tunneling across the air gap that is invariably present during scanning tunneling microscopy (STM) measurements. These results are more compatible with the experimental data.
KEYWORDS: Molecules, Scanning tunneling microscopy, Data modeling, Electrodes, Monte Carlo methods, Electron transport, Reliability, Molecular electronics, Metals, Nanotechnology
Scanning tunneling microscopy measurements of tunneling through molecules adsorbed on a surface have been simulated using a standard empirical model based upon the Wentzel-Kramer-Brillouin method applied to tunneling through a barrier. The Gaussian noise inherent in these experiments has been added to the model data using a Monte Carlo technique. By generating multiple sets of current-voltage curves and fitting these to the model we have evaluated how reliably barrier height can be determined as a function of noise level. The results suggest that for constant percentage standard deviation in the noise greater than 5% the barrier height cannot be determined reliably. At this level, the standard deviation in the estimate of the barrier height is about 10%. Weighted fits give more reliable estimates of the barrier height. If the height of the tip above the molecule is known, so that the fit is only a single parameter the barrier height can be determined reliably even at percentage noise levels as high as 20%. However, in this case unweighted fits must be used otherwise the estimated value deviates by up to 15% from the true value. Data with constant absolute noise give similar results. The effects of experimental resolution have been evaluated in a similar manner and are shown to have a significant influence on the reliability. At a resolution of about 0.1% of full scale the standard deviation in the estimate of barrier height is only about 2% but increases rapidly to 10% for a resolution of about 1%.
The possible role of self-assembled monolayers (SAMs) as the dielectric component of nanoscale capacitors is considered. SAMs of two rather different molecules, α,α’-p-xylyldithiol ('XYL’) and dodecanedithiol ('C12’) were produced on a gold {111} substrate, and characterized with respect to their conductivity. The data were fitted with a double tunnel barrier model, in which the two SAMs are primarily differentiated by barrier height and thickness with that of XYL having a thickness of 1.0 nm and a barrier height of 0.78 eV compared to 1.69 nm and 1.39 eV for C12. The remaining parameters of the model were determined by Monte Carlo optimization. Assuming perfect connection of top and bottom electrodes, the leakage current through the XYL at 1 volt is calculated to be 1.4x105 A/cm2, compared to 2.7x104 A/cm2 through C12. These values are not as low as can be obtained with SiO2 of the same thickness. However, SAMs are readily and precisely produced by simple, low temperature processes, a factor which may allow them a niche in the future.
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