J. Silcox

Non-blinking semiconductor nanocrystals

The photoluminescence from a variety of individual molecules and nanometre-sized crystallites is defined by large intensity fluctuations, known as ‘blinking’, whereby their photoluminescence turns ‘on’ and ‘off’ intermittently, even under continuous photoexcitation. For semiconductor nanocrystals, it was originally proposed that these ‘off’ periods corresponded to a nanocrystal with an extra charge. A charged nanocrystal could have its photoluminescence temporarily quenched owing to the high efficiency of non-radiative (for example, Auger) recombination processes between the extra charge and a subsequently excited electron–hole pair; photoluminescence would resume only after the nanocrystal becomes neutralized again. Despite over a decade of research, completely non-blinking nanocrystals have not been synthesized and an understanding of the blinking phenomenon remains elusive. Here we report ternary core/shell CdZnSe/ZnSe semiconductor nanocrystals that individually exhibit continuous, non-blinking photoluminescence. Unexpectedly, these nanocrystals strongly photoluminesce despite being charged, as indicated by a multi-peaked photoluminescence spectral shape and short lifetime. To model the unusual photoluminescence properties of the CdZnSe/ZnSe nanocrystals, we softened the abrupt confinement potential of a typical core/shell nanocrystal, suggesting that the structure is a radially graded alloy of CdZnSe into ZnSe. As photoluminescence blinking severely limits the usefulness of nanocrystals in applications requiring a continuous output of single photons, these non-blinking nanocrystals may enable substantial advances in fields ranging from single-molecule biological labelling to low-threshold lasers.

Atomic-scale chemical imaging of composition and bonding by aberration-corrected microscopy.

Using a fifth-order aberration-corrected scanning transmission electron microscope, which provides a factor of 100 increase in signal over an uncorrected instrument, we demonstrated two-dimensional elemental and valence-sensitive imaging at atomic resolution by means of electron energy-loss spectroscopy, with acquisition times of well under a minute (for a 4096-pixel image). Applying this method to the study of a La0.7Sr0.3MnO3/SrTiO3 multilayer, we found an asymmetry between the chemical intermixing on the manganese-titanium and lanthanum-strontium sublattices. The measured changes in the titanium bonding as the local environment changed allowed us to distinguish chemical interdiffusion from imaging artifacts.

Simulation of annular dark field stem images using a modified multislice method

Abstract The multislice method of image simulation has been extensively applied to conventional transmission electron micrographs (CTEM), but not yet to scanning transmission electron micrographs (STEM). In this paper the multislice method is adapted for application to the STEM and several examples relevant to the VG-HB501 STEM are presented. Annular dark field images of a (111) crystal silicon substrate supporting a single heavy atom are calculated.

Delocalization in inelastic scattering

How delocalized is an EELS signal? For instance, how far from a silicon atom must the electron probe be before a Si edge can be detected? A crude estimate of a localization length (λθE) suggests a maximum impact parameter of 50 A for a typical plasmon loss. Yet with care, subnanometer resolution plasmon maps can be achieved. The improved resolution cannot be accounted for by dielectric screening as the dielectric screening length diverges for energy losses at and above the plasmon frequency. Niels Bohr [1] offered a classical explanation in 1913 leading to his adiabatic criterion for a cutoff impact parameter bmax = vω, for a fast electron, velocity v, and an energy loss of frequency ω. By starting with a quantum mechanical expression for the energy loss, both the semiclassical limit and surprisingly Bohrs criterion can be recovered and are found to be in excellent quantitative agreement with experiment. We show bmax can be thought of as a “dynamic” screening length and plays the same role as the screening length does in the elastic scattering from a Thomas-Fermi atom. In fact the parallel component of the inelastic scattering has the same form as elastic scattering from a Thomas-Fermi atom and much of our understanding of the role of the detector in annular dark-field imaging can be applied here. However, in inelastic scattering a dramatic improvement in resolution can be obtained with an off-axis detector as suggested by Ritchie and Howie [2]. Here we present experimental evidence of such an effect.

Synthesis and characterization of PbSe quantum dots in phosphate glass

The controlled synthesis of PbSe nanocrystal quantum dots with narrow size distributions was achieved through phase decomposition of the PbSe solid solution in a phosphate glass host. Structural characterization by electron microscopy and x-ray diffraction shows that the dots have mean diameters between 2 and 15 nm. The exciton Bohr radius aB=46 nm in PbSe, so these quantum dots provide unusual and perhaps unique access to the regime of strong quantum confinement. The optical absorption spectra are compared to the predictions of a theoretical treatment of the electronic structure. The theory agrees well with experiment for dots larger than ∼7 nm, but for smaller dots there is some deviation from the theoretical predictions.

Incoherent imaging of zone axis crystals with ADF STEM

Abstract The basic premise of the incoherent imaging model is that the image is the convolution of the incident probe intensity and an appropriate specimen object function. Frozen phonon calculations and multiple experimental defocus series of indium phosphide (100) indicate that annular dark-field (ADF) scanning transmission electron microscope (STEM) imaging follows the incoherent imaging model for crystalline specimens aligned along a zone axis. At the limit of resolution, the ADF STEM signal is very sensitive to incident probe shape, and precise values for the imaging parameters must be known in order to perform quantitative image analysis. The incoherent imaging-model accounts for the subtle effects of the imaging conditions and permits the determination of a specimen object function which is independent of the imaging parameters and therefore simpler to interpret in terms of the actual specimen structure. Identifying and minimizing image artifacts introduced by the imaging conditions is an essential step in quantitative ADF STEM imaging.

Nanofabrication using a stencil mask

We describe tests of a technique to fabricate nanostructures by the evaporation of metal through a stencil mask etched in a suspended silicon nitride membrane. Collimated evaporation through the mask gives metal dots less than 15 nm in diameter and lines 15–20 nm wide. We have investigated the extent of hole clogging and the factors which determine the ultimate resolution of the technique.

Visibility of single heavy atoms on thin crystalline silicon in simulated annular dark-field STEM images

The multislice method, pioneered by Cowley and Moodie, has recently been adapted to simulate annular dark-field scanning transmission electron-microscope (ADF STEM) images. This paper presents a series of calculations using this new approach with experimental parameters appropriate for a VG-HB501 STEM to investigate the visibility of single heavy adatoms on thin crystalline silicon membranes. The tendency for electrons to channel along columns of atoms in crystals can greatly increase the intensity incident on an adatom on the exit surface, thereby increasing the adatom visibility. The simulations indicate that an adatom on the exit surface on a column of crystal atoms is up to three times as visible as an adatom on the entrance surface, and that the adatom remains highly visible as the crystal thickness is increased. Tilting the specimen or displacing the adatom from the column appears to lower the visibility of the adatom dramatically. These calculations suggest that, with the appropriate imaging conditions, a single gold adatom may be visible on at least 235 of (111) silicon.

Detector geometry, thermal diffuse scattering and strain effects in ADF STEM imaging

Abstract Intensities of atomic-resolution Annular Dark Field Scanning Transmission Electron Microscopy (ADF STEM) images of zone-axis-oriented specimens change with defocus at rates that depend on lattice spacing. Thickness and strain effects on the intensities have been demonstrated. In this paper, image simulations (with some experimental basis) are presented that consider the dimensions of the ADF detector. Changing the inner radius of the detector seems to have relatively small effect on the image except to lower the detected intensity. Probe size was explored and a case identified where multiple scattering was important in the image. Thermal diffuse scattering (TDS) is important in high-angle scattering at room temperatures but it does not seem to alter the image appearance markedly. Finally, the image arising from the strain field around a single boron atom has been simulated and the results suggest increased scattering in agreement with observations. This mechanism may be adequate for single impurity atom detection at low temperatures and with special detector angles.

Study of strain fields at a-Si/c-Si interface

The contrast due to a strain field at an amorphous silicon/crystalline silicon (a-Si/c-Si) interface relative to the bulk crystal is studied with a scanning transmission electron microscope equipped with a low angle annular dark field (LAADF) detector and a high angle ADF (HAADF) detector. Experimental observations suggest that strain contrast depends closely on sample thickness and collection angle. For a thin sample ( 150 A) strain contrast is positive in the LAADF image and negative in the HAADF image. Theoretical calculations of the effect of a random strain field are carried out. First, a simple model based on atomic scattering with an extra Debye–Waller factor is employed. It predicts a positive strain contrast in the LAADF image and no contrast in the HAADF image. The simple model fails for the HAADF contrast because it does not consider the propagation process of the electron beam inside the sample. Therefo...