Site-specific laser modification of MgO nanoclusters: Towards atomic-scale surface structuring

Direct neutral metal emission from MgO nanostructures is induced using laser light tuned to excite specific surface sites at energies well below the excitation threshold of the bulk material. We find that near UV excitation of MgO nanocrystalline films and nanocube samples desorbs neutral Mg atoms with hyperthermal kinetic energies in the range of $0.1–0.4\mathrm{eV}$. Our ab initio calculations suggest that metal atom emission is induced predominantly by electron trapping at surface three-coordinated Mg sites followed by electronic excitation at these sites. The proposed general mechanism can be used to control atomic-scale modification of insulating surfaces.

Figure 1

Electron microscopy of MgO nanocubes grown by the CVD technique and MgO thin films grown by the RBD technique: (a) High-resolution TEM image of MgO nanocubes displaying the crystalline structure and morphology with arrows indicating 3C corner sites. (b) SEM image showing the top view of an MgO film (inset). The Bright-field TEM image of two MgO-columns demonstrates that the film consists of highly porous columns with large numbers of low-coordinated sites at the column surface.

Figure 2

Velocity profile and kinetic energy distribution of ${\mathrm{Mg}}^0$ atoms following irradiation with $4.66\mathrm{eV}$ photons and a laser fluence of $4\mathrm{mJ}∕{\mathrm{cm}}^2$ from: (a) MgO nanocube and (b) RBD thin film samples. The insets show the corresponding ${\mathrm{Mg}}^0$ atom kinetic energy distributions obtained by transformation of the velocity profiles; hyperthermal components at $\sim 0.11$, $\sim 0.18$, and $\sim 0.25\mathrm{eV}$ are evident. The dotted line in the velocity profiles represents a Maxwell-Boltzmann fit of the thermal emission component $(T=300\mathrm{K})$.

Figure 3

(Color online) Schematic representation of Mg-atom desorption via biexciton mechanism: (a) the total energy levels of many-electron states involved in the desorption mechanism, and (b) a surface of constant spin-density value calculated for the states of fully relaxed triplet exciton (${\mathrm{Mg}}^{+}\text{−}{\mathrm{O}}^{-}$ configuration) shown on the background of the QM cluster. Thick and thin horizontal lines show the ground and excited states of the system respectively. The processes involved are indicated with arrows.

Figure 4

(Color online) Schematic representation of Mg-atom desorption via electron-plus-exciton mechanism: (a) the total energy levels of many-electron states involved in the desorption mechanism, and (b) a surface of constant spin-density value calculated for the state of an extra electron trapped at the 3C Mg site (${\mathrm{Mg}}^{+}$ configuration) shown on the background of the QM cluster. Other notations are the same as in Fig. 3.