F. Montalenti

Cell cycle effects of gemcitabine.

Gemcitabine (2′,2′‐difluoro‐2′‐deoxycytidine, or dFdC) is a promising anticancer agent with demonstrated clinical activity in solid tumours currently undergoing clinical trials. Despite extensive studies on the biochemical mechanism of action, cell cycle perturbations induced by dFdC have not yet been thoroughly investigated, apart from the expected inhibition of DNA synthesis. The aim of our study was to clarify whether cell population kinetics is a vital factor in the cytotoxicity of dFdC in single or repeated treatments and in the dFdC‐cisplatin combination. Ovarian cancer cells growing in vitro were treated with dFdC for 1 hr in a range of concentrations from 10 nM to 10 μM. Cell kinetics was investigated by DNA‐bromodeoxyuridine flow cytometry, using different experimental protocols to measure either the time course of DNA‐synthesis inhibition or the fate of cells in G1, S or G2M at the time of dFdC treatment or 24 hr later. A modified sulforhodamine B test was used to assess the growth inhibition caused by dFdC given alone or with cisplatin. Although dFdC promptly inhibited DNA synthesis, cytotoxicity on proliferating cells was not specific for cells initially in the S phase. DNA synthesis was restored after a G1 block of variable, dose‐dependent length, but recycling cells were intercepted at the subsequent checkpoints, resulting in delays in the G2M and G1 phases. The activity of repeated treatment with dFdC+dFdC or dFdC+cisplatin was highly dependent on the interval length between them. These results suggest that the kinetics of cell recycling from a first dFdC treatment strongly affects the outcome of a second treatment with either dFdC itself or cisplatin.

Measuring the Complexity of Cell Cycle Arrest and Killing of Drugs: Kinetics of Phase-Specific Effects Induced by Taxol

BACKGROUND Paclitaxel (Taxol) is known to act mainly in mitosis, interfering with microtubule dynamics, but effects on the other cells cycle phases have been reported also. However, a comparative picture of perturbation and killing in the G(1), S and G(2)M phases after drug treatment is lacking. The approach developed by our group tackles the problem of the complexity of cell cycle effects with the aid of a computer program simulating cell cycle progression and new quantities measuring cell-cycle arrest and death. METHODS The program generates data that were compared with those given by absolute cell counts and by different flow cytometry techniques, enabling us to follow the fate of G(1) and G(2)M blocked cells either re-entering the cycle or dying, distinguishing cytostatic and cytotoxic effects. Apoptosis was analyzed in order to refine the description of cytotoxic effects. RESULTS We estimated the number of blocked and dead cells after short-term Taxol treatments in a range of concentrations and post-drug incubation times. G(2)M block was immediately active at low concentrations but was reversible, becoming irreversible only at the highest concentrations. G(1)block became active later, allowing cell cycle progression of cells initially in G(1), but was still active 48 h post-treatment, at intermediate concentrations. S-phase delay was detected after 24 h. The death rate was much higher within G(1)than G(2)M blocked cells. CONCLUSIONS Our analysis unraveled the complexity of cell cycle effects of the drug, and revealed the activity of G(1) checkpoint, hidden by a prompter but less cytotoxic G(2)M block.

Monolithic Growth of Ultrathin Ge Nanowires on Si(001)

Self-assembled Ge wires with a height of only 3 unit cells and a length of up to 2 micrometers were grown on Si(001) by means of a catalyst-free method based on molecular beam epitaxy. The wires grow horizontally along either the [100] or the [010] direction. On atomically flat surfaces, they exhibit a highly uniform, triangular cross section. A simple thermodynamic model accounts for the existence of a preferential base width for longitudinal expansion, in quantitative agreement with the experimental findings. Despite the absence of intentional doping, the first transistor-type devices made from single wires show low-resistive electrical contacts and single-hole transport at sub-Kelvin temperatures. In view of their exceptionally small and self-defined cross section, these Ge wires hold promise for the realization of hole systems with exotic properties and provide a new development route for silicon-based nanoelectronics.

Unexpected Dominance of Vertical Dislocations in High-Misfit Ge/Si(001) Films and Their Elimination by Deep Substrate Patterning

An innovative strategy in dislocation analysis, based on comparison between continuous and tessellated film, demonstrates that vertical dislocations, extending straight up to the surface, easily dominate in thick Ge layers on Si(001) substrates. The complete elimination of dislocations is achieved by growing self-aligned and self-limited Ge microcrystals with fully faceted growth fronts, as demonstrated by AFM extensive etch-pit counts.

Strain-induced ordering of small Ge islands in clusters at the surface of multilayered Si–Ge nanostructures

Classical molecular-dynamics simulations based on the Tersoff potential are used to compute at the atomic level the strain-induced potential well generated at the surface of the capping layer by a buried, three-dimensional Ge island on Si(001). A simple model is outlined in order to predict the configurational arrangement for the nucleation of small Ge islands in such a potential well. The theoretical predictions are compared with atomic force microscope images of multilayered SiGe nanostructures grown by chemical vapor deposition. The cluster configuration is shown to be strongly dependent on the capping layer thickness, and to closely mimic the behavior predicted by the model.

Fine control of plastic and elastic relaxation in Ge/Si vertical heterostructures

We present a theoretical investigation of plasticity onset and strain relaxation in Ge on Si pillar-like, vertical heterostructures (VHEs). By means of linear elasticity theory solved by Finite Element Methods, we determine the critical thickness hc for the insertion of a 60° dislocation in Si1–xGex/Si VHEs as a function of their lateral extension. Then, we quantify the effect of inserting one or more buffer layers in further delaying plasticity when growing a Ge-pure layer on top of the VHEs. The presence of intermediate layers of suitable Ge content allows for the formation of fully coherent structures up to the micron scale. The optimal thickness of one or multiple buffers to avoid dislocations is also discussed.

Intermixing in heteroepitaxial islands: fast, self-consistent calculation of the concentration profile minimizing the elastic energy

We present a novel computational method for finding the concentra- tion profile which minimizes the elastic energy stored in heteroepitaxial islands. Based on a suitable combination of continuum elasticity theory and configura- tional Monte Carlo, we show that such profiles can be readily found by a simple, yet fully self-consistent, iterative procedure. We apply the method to SiGe/Si islands, considering realistic three-dimensional shapes (pyramids, domes and barns), finding strongly non-uniform distributions of Si and Ge atoms, in qualitative agreement with several experiments. Moreover, our simulated selective-etching profiles display, in some cases, a remarkable resemblance to the experimental ones, opening intriguing questions on the interplay between kinetic, entropic and elastic effects.

Self-assembled GaAs islands on Si by droplet epitaxy

We presented an innovative fabrication technique for the self-assembly of GaAs islands on Si substrates by droplet epitaxy. The islands show highly tunable density (from 107 to some 109 islands/cm2) and size (from 75 to 250 nm), and small size dispersion (below 10%). The islands, made by single relaxed crystals with lattice parameters close to the GaAs bulk, show well defined shapes, with a high aspect ratio. The low thermal budget required for the island self-assembly, together with the high scalability of the process, make these islands good candidates for local artificial substrates or local strain sources with the required lattice parameters, band alignment, and crystalline quality as now required for the implementation of high quality devices on Si.

Applying Accelerated Molecular Dynamics to Crystal Growth

We discuss how recent accelerated molecular dynamics techniques can be applied to crystal growth studies. Focusing our attention on the temperature accelerated dynamics (TAD) method, we show that at low temperatures the accessible time scale can be several orders of magnitude greater than with ordinary molecular dynamics. This allows us to match experimental deposition rates, as shown by preliminary results for the Cu/Cu(100) system.

Highly Mismatched, Dislocation-Free SiGe/Si Heterostructures

Defect-free mismatched heterostructures on Si substrates are produced by an innovative strategy. The strain relaxation is engineered to occur elastically rather than plastically by combining suitable substrate patterning and vertical crystal growth with compositional grading. Its validity is proven both experimentally and theoretically for the pivotal case of SiGe/Si(001).