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II.Nanocrystal Quantum Dots  >> 1. Integrated Epitaxical and Colloidal Nanostructure - Study of Energy Transfer


Integrated Epitaxical and Colloidal Nanostructure - Study of Energy Transfer


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

In this study, the integrated colloidal and epitaxical nanostructure is formed by adsorbing a sub-monolayer of InAs/ZnSe core/shell nanocrystal on MBE prepared GaAs substrate with a InGaAs near surface quantum well (NCQD). Figure 4 shows the room temperature PL of InAs/ZnSe NCQDs deposited on such near surface QW (red) and on glass (green) substrates with excitation at 900nm (panel a) and at 850nm (panel b). At room temperature, the former is below and the latter above the GaAs band gap (872nm). The PL response from NCQDs on glass (fig.2, green) for excitation at either wavelength is essentially the same with a peak at 1220nm.


Figure 1. Shows schematic of InAs/ZnSe core/shell nanocrystal quantum dots with its surfactant in contact with GaAs substrate with buried near surface InGaAs QW.


Figure 2. Room temperature PL of NCQDs on different substrates for excitation at (a) below (9000nm) and (b) above (8500) GaAs band gap. In both panels, on glass (green); on NSQW containing GaAs (Red).




However, for the NSQW containing GaAs substrates, the NCQD PL peak intensity at 1220nm is seen to drop by a factor of ~5 as the excitation goes from below to above GaAs band gap. This drop in PL intensity is also manifested in the corresponding PLE spectra (figure 3) for detection at the peak of the NCQD PL. Note that the PLE of NCQDs on glass shows only small variation in the excitation wavelength region of 850nm-1000nm, manifesting the near constancy of the absorption coefficient of the NCQDs in this regime. By contrast, the PLE intensity of the NCQDs on GaAs substrate with NSQW is seen to exhibit step-like drop as the excitation energy changes from below to above the GaAs band gap. The reduction in PL intensity of the NCQDs on NSQW containing GaAs substrate at excitation energies above the GaAs band gap implies that the substrate is quenching the luminescence of the adsorbed NCQDs due to energy transfer from the NCQDs into the substrate. This may be understood with the help of the energy band diagram (fig.4) for InAs/ZnSe NCQD on NSQW containing GaAs substrate. GaAs has a room temperature bandgap of 1.42eV (872nm), larger than the ~1eV (1100nm) ground state transition energy for our InAs/ZnSe NCQDs. Remember that excitation (absorbtion) in the NCQDs is occurring in their higher energy excited states (see fig.4). Once excited, relaxation processes of the electron-hole pair (exciton) created in NCQDs come into play. At low excitation power density, following the creation of electron-hole pair in the NCQDs, the eventual ground state luminescence from the NCQDs may be approximated to be controlled by a two-step process: excitons first relax thermally into the ground state, and then recombine radiatively to emit photons. If NCQDs are excited with a photon energy lower than the GaAs bangap, no transfer to the substrate can occur during the exciton thermal relaxation to its ground state. By contrast, when excited with a photon energy higher than the GaAs bandgap, energy transfer into the substrate can occur (if empty states of matching energy exist) that competes with thermal relaxation to the NCQD ground state. Indeed, the opening of the energy transfer pathway thus lowers the ground state luminescence yield of the NCQDs which is manifested as the step feature in the NCQD PLE spectrum. This is the first experimental demonstrate of such an energy transfer from the nanocrystal quantum dots into the underlining semiconductor substrate.

Figure 3. Room temperature PLE of NCQDs deposited on: glass (broken curve), NSQW containing GaAs (Red). Detection is at 1220nm (NCQD PL peak). Figure 4 Schematic energy level alignment of the NC/substrate system depicted in figure 1.  
[1] S. Lu, A. Konkar and A. Madhukar, Nano Letter (submitted)


II.2 TEM Study
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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