Steve P. Wilks

Single cell nanoparticle tracking to model cell cycle dynamics and compartmental inheritance

Single cell encoding with quantum dots as live cell optical tracers for deriving proliferation parameters has been developed using modelling to investigate cell cycle and proliferative outputs of human osteosarcoma cells undergoing mitotic bypass and endocycle routing. A computer-based simulation of the evolving cell population provides information on the dilution and segregation of nanoparticle dose cell by cell division and allows quantitative assessment of patterns of division, at both single cell and including whole population level cell cycle routing, with no a-priori knowledge of the population proliferation potential. The output therefore provides a unique mitotic distribution function that represents a convolution of cell cycle kinetics (cell division) and the partitioning coefficient for the labelled cell compartment (daughter-daughter inheritance or lineage asymmetry). The current study has shown that the cellular quantum dot fluorescence reduced over time as the particles were diluted by the process of cell division and had the properties of a non-random highly asymmetric event. Asymmetric nanoparticle segregation in the endosomal compartment has major implications on cell-fate determining signaling pathways and could lead to an understanding of the origins of unique proliferation and drug-resistance characteristics within a tumour cell lineage.

Factors that determine and limit the resistivity of high-quality individual ZnO nanowires

Knowing and controlling the resistivity of an individual nanowire (NW) is crucial for the production of new sensors and devices. For ZnO NWs this is poorly understood; a 10(8) variation in resistivity has previously been reported, making the production of reproducible devices almost impossible. Here, we provide accurate resistivity measurements of individual NWs, using a four-probe scanning tunnelling microscope (STM), revealing a dependence on the NW dimensions. To correctly interpret this behaviour, an atomic level transmission electron microscopy technique was employed to study the structural properties of the NWs in relation to three growth techniques: hydrothermal, catalytic and non-catalytic vapour phase. All NWs were found to be defect free and structurally equivalent; those grown with a metallic catalyst were free from Au contamination. The resistivity measurements showed a distinct increase with decreasing NW diameter, independent of growth technique. The increasing resistivity at small NW diameters was attributed to the dominance of surface states removing electrons from the bulk. However, a fundamental variance in resistivity (10(2)) was observed and attributed to changes in occupied surface state density, an effect which is not seen with other NW materials such as Si. This is examined by a model to predict the effect of surface state occupancy on the measured resistivity and is confirmed with measurements after passivating the ZnO surface. Our results provide an understanding of the primary influence of the reactive nature of the surface and its dramatic effect on the electrical properties of ZnO NWs.

A quantitative study of the formation of breath figure templated polymer materials

Self-assembled ordered arrays of pores are formed when a polymer-solvent solution is deposited in the presence of a humid airflow. These structures can be used as biological scaffolds, photonic bandgap materials and microfluidic beakers. Despite a wealth of material in the published literature regarding the growth of these structures, the dynamics of the process have received little attention from a quantitative perspective. Before the self-assembly mechanism can be understood, it is important to first look at the co-existent driving conditions. Here we develop such a computational model to describe this casting process, which finds excellent agreement with published data. The solvent evaporation profile is found to be near-linear for the majority of the casting process. During this stage a steady-state thermal system exists. The model shows that a humidity threshold exists for the creation of self-assembled structures, with threshold values which find excellent agreement with the literature. Measurement estimates taken of condensate deposition on to the polymer film match the order of magnitude and trend of computational values. Although not given attention in the literature before, slide thickness is shown to be a crucial parameter in this process. The model is able to identify the critical parameters in this system and show which should be controlled and specified to enable experimental results to be repeated. The ability of this model to accurately match experimental results sets it up as the basis for development of a full approach to capture the dynamics of the self-assembly formation process.

GaN Cleaning by Ga Deposition, Reduction and Re‐Evaporation: An SXPS Study

The Ga deposition, reduction, and re-evaporation technique commonly used to produce clean n-GaN surfaces and Ag–GaN interface formation on the resultant surface, have been investigated by Soft X-ray Photoelectron Spectroscopy (SXPS) and current–voltage measurements. SXPS studies have indicated that Ga deposition produces a band-bending of ΔEk = + 1.0 eV to higher kinetic energy. Our results show this shift to be a partially reversible process: re-evaporation of the deposited Ga resulted in a Fermi shift of ΔEk = — 0.6 eV to lower energy. Ag deposition did not cause any further Fermi shift, indicating that the Fermi level is pinned (2.2 ± 0.2) eV above the valence band edge, possibly as a consequence of the cleaning procedure itself. Current voltage (I–V) measurements have shown a barrier height of 0.77 eV and an ideality factor of 1.6. Metal induced gap states and the unified defect model are discussed as possible barrier formation mechanisms.

Controlling the Electrical Transport Properties of Nanocontacts to Nanowires

The ability to control the properties of electrical contacts to nanostructures is essential to realize operational nanodevices. Here, we show that the electrical behavior of the nanocontacts between free-standing ZnO nanowires and the catalytic Au particle used for their growth can switch from Schottky to Ohmic depending on the size of the Au particles in relation to the cross-sectional width of the ZnO nanowires. We observe a distinct Schottky to Ohmic transition in transport behavior at an Au to nanowire diameter ratio of 0.6. The current-voltage electrical measurements performed with a multiprobe instrument are explained using 3-D self-consistent electrostatic and transport simulations revealing that tunneling at the contact edge is the dominant carrier transport mechanism for these nanoscale contacts. The results are applicable to other nanowire materials such as Si, GaAs, and InAs when the effects of surface charge and contact size are considered.

A surface extended X-ray absorption fine structure study of tellurium adsorbed onto Si(100)

Abstract The adsorption of tellurium on Si(100) has been studied using surface extended X-ray adsorption fine structure (SEXAFS) and X-ray standing wave spectroscopy (XSW). This particular system is of interest due to its potential applicability in the surfactant aided growth of CdHgTeCdTeSi(100) based infra-red detectors. The Te Si (100) structure was generated by depositing a thick layer (∼ 100 A) of CdTe onto a clean Si (2 × 1) double domain surface, and annealing the sample to 350°C. This resulted is a ∼ 1 ML Te terminated surface where the (2 × 1) reconstruction was lost in favour of a (1 × 1) symmetry. X-ray absorption of the Te L3 edge (E = 4341 eV), with a photon energy range of 4440–4700 eV, was probed using a total yield detection scheme. The SEXAFS results indicated that the Te atoms sat in 2-fold bridge sites directly above a fourth layer Si atom. The corresponding bond length was measured to be 2.52 ± 0.05 A . The XSW measurements of the (400) reflection gave a coherent position of 1.63 ± 0.03 A and a coherent fraction of 0.65. This is consistent with the breaking of the SiSi dimers and thus could be an example of the phenomena of adsorbate-induced dereconstruction of the surface. These results are compared with those of Bennet et al. who examined a similar system using soft X-ray photoemission (SXPS) and the STM study of Yoshikawa et al.

ZnO nanowires with Au contacts characterised in the as-grown real device configuration using a local multi-probe method

We demonstrate here a method using a multi-probe UHV instrument to isolate and measure individual metal contacts controllably fabricated on the tips of free standing ZnO nanowires (NWs). The measurements show Au can form reliable Ohmic and rectifying contacts by exercising control over the surface properties. In the as-grown state the Au contacts display low-resistance characteristics which are determined by the adsorbed species and defects on the NW surface. Subjecting the NWs to an oxidising agent (H2O2) increases the surface potential barrier creating more rectifying contacts. These developments are crucial for controllable NW array devices.

The effect of interface roughness on multilayer heterostructures

Semiconductor devices which utilize the quantum confinement of charge carriers inherently employ material layers thin enough that even monolayer interface roughness has an effect on performance. We present a method for including the effect of interface roughness on the calculation of electron energy levels and wavefunctions by solving Schrodinger’s equation across the interface between semiconductor layers. Interface roughness is approximated by considering a supplementary interface in addition to the idealized perfectly flat interface. The position of the second interface is considered to be a probabilistic distribution with a mean corresponding to the position of the perfect case. Using Green’s theorem and the appropriate reciprocity relations, we deduce a correction to the reflection and transmission probabilities of an electron incident upon a rough material interface. The procedure is presented in terms of a transfer matrix algorithm to facilitate use in existing electron reflection transmission prob...

Direct real-time observation of catastrophic optical degradation in operating semiconductor lasers using scanning tunneling microscopy

Cross-sectional scanning tunneling microscopy is performed on operating semiconductor quantum well laser devices to reveal real time changes in device structure. Low and nominally doped capping regions adjacent to the quantum well active region are found to heat under normal operating conditions. The increase in anion-vacancy defect formation and the generation of surface states pins the Fermi level at the surface and begins the process of catastrophic optical degradation which eventually destroys the device. The technique has implications for the study of defect generation and in-operation changes in all nanostructures.

Computational simulation of optical tracking of cell populations using quantum dot fluorophores

Quantum dot fluorophores provide a photo and bio-stable optical marker signal well suited to the tracking of lineage within large cell populations over multiple generations. We have used a Monte Carlo algorithm to model the process of dot partitioning and dilution by cell mitosis. A Genetic Algorithm was used to compare simulated and experiment quantum dot distributions, which shows that the dot fluorescence is divided with a stochastic variation about an asymmetric mean split ratio.