Manipulation and electrical characterization of CdSe nanocrystals by atomic force microscopy E. Tranvouez It is likely that nanocristals play a major role in the near future for both electronics and optoelectronics. Thanks to their dimensions and the related properties, they could soon be involved in applications such as single photon emission source. For this reason, the study of the photonic emission capabilities of CdSe nano-crystal by electrical excitation needs to be addressed. My postdoctoral work was to carry out the first step of this study by a) Developing and studying of the manipulation of CdSe nanocrystals (or more precisely CdSe Nanorod) by atomic force microscopy (AFM) and b) performing electrical characterization of CdSe nanocrystals by AFM to define their electro-luminescence capabilities.
Drift correction and automated manipulation 1 We have developed a simple algorithm to overcome the problem of thermal drift in AFM operating under ambient conditions. Using our method, the AFM tip can remain above a 5 nm-high and 50 nm-long CdSe nanorod for more than 90 minutes despite the thermal drift present (6 nm/min) (see figure 1). We have adapted our drift compensation technique to the AFM manipulation of nano-objects. With this method we have the possibility to precisely control both the position of the AFM tip relative to the nano-object and the applied force during the manipulation. We used this capability to study2, characterize, and control (see figure 2) the manipulation of CdSe colloidal nanorods lying horizontally on a highly oriented pyrolytic graphite (HOPG) surface. This surface - nano-object combination was chosen for their anisotropic interaction that leads to nanorod auto-organization (following three axes)3. We show, during this study, that this anisotropic interaction produces an anisotropy in the nanorod movement and that as a result it is not possible to manipulate the nanorod in a direction parallel to its axis. From deflection measurements during manipulation and simulations we estimate the lateral force needed to move the nanorod and consequently the static friction between the nanorod and the graphite surface.
Electrical characterization and electro-luminescence Electrical excitation of CdSe luminescence requires a conductive surface that is transparent for the emission wavelength of the nanorod and possesses a wide gap (to insure of the excitation confinement). We chose, on this basis, the (100) hydrogenated natural diamond surface that also offers a p-doped surface conductivity by the holes (the injection of holes could benefit the electrical excitation). The lack of a proper study of this surface and more precisely on the hydrogenation process convinced us to perform a complete study of this process4. We show and quantify by using a combination of AFM and scanning tunneling microscopy (STM) the smoothing of the surface and the growth of twinning crystals during 1
Active drift compensation applied to nanorod manipulation with an atomic force microscope E. Tranvouez, E. Boer-Duchemin, G. Comtet, and G. Dujardin Rev. Sci. Instrum. 78, 115103 (2007) 2 Manuscript in preparation 3 R. Bernard, G. Comtet, G. Dujardin, A. J. Mayne, V. Huc, and H. Tang, Phys. Rev. B 75, 045420 (2007).
hydrogenation. We also characterize the conductivity of this surface by using the electrical capability of AFM. We show that this conductivity is sufficient for the study of CdSe nanocrytal conductivity and excitation. The study of the electrical conduction of CdSe nanocrystals is currently in progress.
Figure 1 Thermal drift compensation demonstration: the AFM tip remains above a CdSe nanorod(circled) on a graphite surface for more than 90 minutes. a) AFM image of the nanorod just before b) AFM image taken 90 minutes later, The line trace shows the path of the tip during tracking. The fact that the path ends at the final position of the nanorod demonstrates that the tip has indeed remained above the nanorod.
Figure 2 : Translation and rotation of a CdSe nanorod on a graphite surface. a) AFM image before translation. b) AFM image after translation c) AFM image before rotation. d) AFM image after rotation. For all images, the line traced on top of the rod represents the nanorod's position before the manipulation and the arrow shows the tip trajectory during manipulation.
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“Investigation of the influence of plasma hydrogenation on diamond surfaces; from the microscopic to atomic scale” E. Tranvouez, R. Bernard, T Vanderbruggen, M. Scheele, E. Boer-Duchemin, A.J. Mayne, G. Comtet and G. Dujardin To be submitted to Journal of Applied Physics
