Excitation of Surface Excitons in the Nanoscale Calcium Oxide Surface
EMSL Project ID
21596
Abstract
Optical absorption and photoluminescence emission properties of dehydroxylated MgO and CaO CVD-grown nanocrystals have recently been undertaken with respect to particle morphology and size. On MgO nanocubes with pronounced corner and edge features two emission bands at 3.4 and 3.3 eV result from the excitation of 4-coordinated surface O4C2-anions in edges at 5.4 eV and of regular oxygen-terminated corners at 4.6 eV, respectively. Complementary studies at EMSL using surface laser desorption techniques have comfirmed that surface excitons can be directly excited in both CVD-grown and reactive ballistically-deposited (RBD) MgO thin films at oxygen- and magnesium-terminated corners with 4.6 eV radiation.
In the particular case of dehydroxylated CaO CVD-grown nanoparticles, emission spectra were recorded with excitation energies between 5.2 and 3.7 eV. In fact, 3.7 eV corresponds to the minimum excitation energy that is associated with significant photoluminescence emission. For the entire excitation energy range, there is only one emission band with a maximum at 3.0 eV detectable, the shape of which is also excitation energy independent. The associated excitation spectrum reveals one asymmetric curve, which suggests that more than one excitation process contributes to the photoluminescence emission at 3.0 eV. In fact, the optical absorption spectrum can be decomposed into two constituents at 4.2 and 4.7 eV.
At EMSL surface exciton excitation at 4.7 eV and subsequent hyperthermal O-atom desorption have recently been observed in RBD-grown CaO thin films. Atomic hyperthermicity is an indicator of surface species desorption. Suprising, no O-atom desorption is observed upon excitation at 4.2 eV. What is observed is hyperthermal, neutral metal Ca-atoms desorption. This exciting result indicate that selective photoexcitation may allow atomic-scale sculpturing of the metal oxides surface.
This collaboration envisions the use of both nanoporous RBD CaO thin films and the Austrian CVD-grown CaO nanoparticles in the EMSL 1221 laboratory for laser desorption measurement of low-coordinated sites. EPR/ESR and PL studies of RBD type samples would then be compared to CVD-grown CaO in our Austrian lab to help elucidate the formation and role of low-coordinate oxygen excitation and ionization on CaO surfaces.
Project Details
Project type
Exploratory Research
Start Date
2006-09-13
End Date
2007-09-13
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members