International audienceZnO nanowires grown in liquid phase are considered as promising building blocks for a wide variety of optical and electrical devices. However, their structural morphology is still limited by the lack of understanding of their growth mechanisms. We have systematically investigated the effects of orientation and polarity of ZnO monocrystals acting as substrates on the formation mechanisms of ZnO by chemical bath deposition. Under identical growth conditions, two-dimensional layers develop on nonpolar m- and a-plane ZnO monocrystals. In contrast, nanowires form on O-polar c-plane ZnO monocrystals, while more complex nanostructures including nanowires grow on Zn-polar c-plane ZnO monocrystals. All of the structures have homoepitaxially nucleated. Very specifically to chemical bath deposition, both O- and Zn-polar c-planes are found to be active, and no polarity inversion domain boundary is observed on O-polar c-plane ZnO monocrystals, allowing the growth of O-polar ZnO nanowires. These findings reveal the crucial role of crystal orientation and polarity in the growth of ZnO nanowires in liquid phase similarly to their growth in vapor phase. They further cast a new light on the general understanding of the growth of ZnO nanowires and enable the revisiting of their formation mechanisms in liquid phase on seed layers consisting of ZnO nanoparticles

International audienceMastering the structural ordering of ZnO seed layers by sol–gel process in terms of ultrathin thickness (i.e, <10 nm), strong c-axis texture, low mosaicity, low porosity, and low roughness is a critical challenge for the formation of well-ordered ZnO nanowires in solution. The effects of the solution concentration, of the withdrawal speed, and of the annealing process on the formation mechanisms of ZnO seed layers deposited by single dip process are revealed. The size and density of primary clusters in the sol are found to govern the evolution of the film thickness and nanoparticle average diameter through the solution concentration. The Landau–Levich theory modeling the dragging process accounts for the evolution of the film thickness only before annealing and over a reduced range of withdrawal speeds. The texture mechanisms along the c-axis are driven by particle/particle interactions during annealing and explained in the light of thermodynamic considerations. They are further determined locally by electron backscattered diffraction. Importantly, an alternative annealing process under argon flux is specifically developed for sol–gel process and is shown to form remarkably well-textured, compact ZnO seed layers with a very low mosaicity and porosity as well as a thin thickness as small as 10 nm. These ZnO seed layers lead to the growth of well-ordered ZnO nanowires by chemical bath deposition with a remarkable mean tilt angle smaller than 6° as deduced by X-ray pole figures. These findings represent a significant step toward the more efficient integration of ZnO seed layers grown by sol–gel process into ZnO nanowire-based devices

InAs nanostructures were grown on In0.52Al0.48As alloy lattice matched on InP(001) substrates by molecular beam epitaxy using specific growth parameters in order to improve island self-organization. We show how the change in InAs surface reconstruction via growth temperature from (2×4) to (2×1) and/or the use of InAlAs initial buffer surface treatments improve the island shape homogeneity (either as quantum wires or as quantum dots). Differences in island shape and in carrier confinement are shown by atomic force microscopy and by photoluminescence measurements, respectively. We point out that such shape amendments induce drastic improvements to island size distribution and discernible changes in photoluminescence properties, in particular concerning polarization.

We show how the height dispersion of self-organized InAs/InP(001) quantum islands emitting at 1.55 μm was reduced by optimizing the epitaxial growth parameters. Low height dispersion was obtained when the InAs deposit thickness was much greater than the critical thickness for two-dimensional/three-dimensional growth mode transition, and when adatom surface diffusion was favored by increasing the growth temperature or reducing the arsenic pressure during the InAs growth. When these growth conditions are not respected, the multicomponent photoluminescence spectrum obtained is explained through the common interpretation of island height varying with monolayer fluctuation. In optimized growth conditions, the multicomponent spectrum obtained is interpreted as emission from fundamental and excited levels of InAs islands with low height dispersion. Transmission electron microscopy (TEM) imaging shows that these InAs islands are stick-like, 50–100 nm in length and 22±1.2 nm in width. Cross-sectional TEM reveals flat islands, shaped like truncated pyramids, with a very homogeneous height measured at 2.4 nm. A fundamental level linewidth of 22 meV at 8 K is associated to this very narrow height distribution. Such low photoluminescence linewidth values are believed to be mainly due to the propensity of the InAs/InP(001) system to produce flat InAs islands with discrete height fluctuation.

We have studied the influence of SiO 2 surface properties on the nucleation and growth of silicon quantum dots ͑Si-QDs͒ deposited by SiH 4 low-pressure chemical vapor deposition ͑LPCVD͒. First, the effect of siloxane groups ͑Si-O-Si͒ strain at the SiO 2 surface layer, characterized by Fourier transform infrared ͑FTIR͒ spectroscopy, is studied. We evidenced an increase of Si-QD nucleation with the strain of siloxane groups in the SiO 2 substrate layer. Second, the Si-QD nucleation strongly depends on the surface silanol group ͑Si-OH͒ density. This density, controlled by chemical and thermal treatments, is measured by multiple internal reflexion ͑MIR͒ FTIR. Very high Si-QD densities larger than 10 12 /cm 2 are obtained on highly hydroxylated SiO 2 .During the last few years, silicon quantum dots ͑Si-QDs͒ have been studied for nanoelectronics applications. Their unique physical properties, size confinement effect, and coulomb blockade phenomena make Si-QDs suitable for use in new silicon-based devices like single electron transistors 1 or quantum dot floating gate memories. 2 For room-temperature operation of such devices, nanometric size silicon dots ͑Ͻ10 nm͒ are required.Low-pressure chemical vapor deposition ͑LPCVD͒ is a good way to obtain Si-QDs for industrial applications because of its metal-oxide field effect transistor ͑MOSFET͒ technology compatibility. By controlling the early stages of the Si film growth, silicon crystallites of nanometer size ͑5 nm͒ are obtained. 3 It has been shown that Si-QDs elaborated by SiH 4 LPCVD could exhibit coulomb blockade at room temperature 4 but to successfully integrate Si-QDs in devices their alignment must be controlled. Results were recently obtained by Baron et al. 5 who deposited an ordered array of Si-QDs by SiH 4 LPCVD on a substrate realized by wafer bonding with a periodic strain field at the surface.In order to obtain operating devices, size, size uniformity, and Si-QDs density must also be controlled with great precision and reproducibility. For floating gate memory applications, densities between 10 11 and 10 12 /cm 2 are required. To fabricate a single electron transistor with a lateral current transport, the spacing between dots should be lower than 2 nm. Typically, for a dot size of 5 nm, these conditions lead to a density of 3 ϫ 10 12 Si-QDs/cm 2 . 6 Si-QD size and density are piloted by pressure and temperature conditions. The chemical nature of the substrates 3 and its physical properties such as stress, roughness, or defects can play an important role in silicon nucleation. Voutsas and Hatalis 7 showed that the chemical properties of the SiO 2 surface also strongly affect the first stages of silicon deposition.In this paper, we separately investigated the influence of ͑i͒ the strain of siloxane ͑Si-O-Si͒ groups and ͑ii͒ the surface silanol ͑Si-OH͒ density on thermally grown SiO 2 layers on Si-QD nucleation by SiH 4 LPCVD. These characteristics are controlled by the oxidation process and postoxidation chemical and thermal treatments, respectively. Experim...

An atomic force microscopy (AFM) tip has been used to manipulate silicon nanocrystals deposited by low-pressure chemical vapour deposition on thermally oxidized p-type Si wafer. Three nanomanipulation methods are presented. The first one catches a nanocrystal with the AFM tip and deposits it elsewhere: the tip is used as an electrostatic ‘nano-crane’. The second one simultaneously manipulates a set of nanocrystals in order to draw well-defined unidimensional lines: the tip is used as a ‘nano-broom’. The third one manipulates individual nanocrystals with a precision of about 10 nm using both oscillating and contact AFM modes. Switching from strong interaction forces (chemical) to weak ones (van der Waals, electrostatic or capillarity) is the basis of these manipulation methods. We have applied the second method to connect two electrodes drawn by e-beam and lift-off with a 70 nm long silicon nanocrystal chain. Current versus voltage characterization of the nanofabricated device shows that the increase in nanocrystal density gives rise to conduction between the connected electrodes. Resonant tunnelling effects resulting from Si nanocrystal (nc-Si) multiple tunnel junctions have been observed at 300 K. We also show that offset charges directly influence the position of the resonant tunnelling peaks. Finally, the possibility of manipulating nc-Si with a diameter of around 5 nm is shown to be a promising way to fabricate single electron devices operating at room temperature and fully compatible with silicon technology.

Steady‐state and time‐resolved photoluminescence of silicon nanoparticles dispersed in low‐polar liquids at above room temperature is studied. The roles of low‐polar liquids as well as mechanisms responsible for their temperature‐dependent photoluminescence are discussed. The thermal sensitivity of the photoluminescence is estimated and application of the nanoparticles as nanothermometers is proposed. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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