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II.Nanocrystal Quantum Dots  >> 2. TEM Studies


Structural nature of InAs NCQDs on GaAs(001)


This work is supported by: DURINT-01 AFOSR Grant No. F49620-01-1-0474

Both the integration schemes noted above require deposition of the NCQDs on the semiconductor substrate. Therefore we would like to know the structural and chemical nature of the NCQDs in the as-deposited state. Given the NCQD size the only viable technique for their studies is high-resolution transmission electron microscopy (HRTEM). Most of the previous TEM studies of NCQDs have been performed when the NCQDs are deposited on amorphous substrates such as the image shown in fig. 2 in the introduction section. HRTEM studies on single crystal semiconductor substrate are not a simple matter. For imaging in plan-view geometry, the substrate thickness needs to be below ~ 20nm to be able to image contrast from NCQDs. Due to the fragility of the NCQDs, conventional techniques for semiconductor thinning based on ion-beam sputtering are not appropriate. We have therefore developed and applied new techniques for the specimen preparation based exclusively on chemical etching that are benign and enable maintaining the integrity of the integrated hybrid structures. A detailed account can be found in reference 1. Figure 1(a) and (b) show a low magnification TEM of an ensemble of InAs NCQDs on GaAs(001) and the corresponding electron diffraction pattern, respectively. The TEM image shows that the InAs NCQDs for dense close-packed patches. The ring pattern from the NCQD ensemble in fig. 1(b) reveals that the NCQDs are randomly oriented on the substrate.


Figure 1 (a) Plan-view TEM image of InAs NCQDs on GaAs(001),
(b) corresponding electron diffraction pattern.      



Figure 2 shows a HRTEM image of a single NCQD. Lattice fringes from the {111} InAs planes are clearly observed from the NCQD. The faint vertical lines in the background are the lattice fringes from the {220} GaAs substrate planes. Notice that there is a dark halo, ~1-1.5nm thick, surrounding the InAs NCQD. This thickness is consistent with the expected thickness of the tri-octyl phosphine (TOP) surfactant molecules that decorate the surface of the NCQDs in the as-synthesized state. These molecules play a dual role: (1) prevent coagulation of the NCQDs in the solution and (2) provide passivation of the NCQD surface states. Their presence suggests that there is no covalent bonding between the NCQDs and the substrate thus explaining their lack of any preferred orientation. For our intended application, the presence of TOP is indeed necessary so that the integrity of the NCQD luminescence is maintained in the integrated hybrid structures.


Figure 2. HRTEM image on an isolated InAs NCQD on GaAs(001).
1. "Semiconductor nanocrystal quantum dots on single crystal semiconductor substrates: High resolution transmission electron microscopy", A. Konkar, S. Lu, A. Madhukar, S. M. Hughes, and A. P. Alivisatos (accepted, to appear in the May 2005 issue of Nano Lett.).


II.1 Study of Energy Transfer
II.3 Integrated Hybrid Nanostructures: Overgrowth on InAs NCQDs

DURINT Project
    - Nanocrystal/Eptaxial 2D integration
    - Surface Modification (Chemical & Biochemical)
    - Nanoscale Simultaneous Morphological & Optical Imaging

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