Cd-based II-VI semiconductor nanostructures produced by buffer-layer-assisted growth: Structural evolution and photoluminescence

II-VI semiconductor nanostructures exhibit interesting optical properties due to quantum confinement of their charge carriers. Here, we discuss the assembly of nanostructures of $\mathrm{CdS}$, $\mathrm{CdSe}$, and $\mathrm{CdTe}$ using buffer-layer-assisted growth with Xe buffers. Both compact clusters and ramified wires can be synthesized by varying the Xe buffer layer thickness. Analysis of the nanostructure size distributions and densities makes it possible to calculate their diffusion parameters on the desorbing Xe. Clear differences in the effective activation energies for diffusion for $\mathrm{CdS}$, $\mathrm{CdSe}$, and $\mathrm{CdTe}$ can be attributed to differences in London dispersion interactions. Photoluminescence measurements indicate changes from 3D to 2D confinement as compact particles are replaced by ramified wires. Laser power dependent measurements yield the low temperature exciton lifetime, and temperature dependent measurements indicate that optical phonons play a dominant role in the decay of the signal above $50\mathrm{K}$ and defect states play a dominant role below $50\mathrm{K}$.

Figure 1

TEM images of clusters formed by depositing $2\mathrm{\AA }$ $\mathrm{CdTe}$ on Xe of $4–20\mathrm{ML}$. For thin buffers, the structures are compact. Ramified islands are produced with thicker buffers. They show increasing branch lengths and widths of $\sim 2\mathrm{nm}$. The scale bar is $80\mathrm{nm}$ in each image.

Figure 2

HRTEM images showing single crystals of (a) $\mathrm{CdS}$, (b) $\mathrm{CdSe}$, and (c) $\mathrm{CdTe}$ that exhibit the zinc blende structure. The polycrystalline nature of the aggregated particle in (d) is evident from the presence of planes with different orientations in the same particle. The $\mathrm{CdS}$ crystallite enclosed in (d) measures $7\mathrm{nm}$ in length and $5\mathrm{nm}$ across the center, and shows a (111) orientation. It was obtained by growing $1\mathrm{\AA }$ $\mathrm{CdS}$ on $10\mathrm{ML}$ Xe. All scale bars are $1\mathrm{nm}$ in length.

Figure 3

Particle density vs buffer thickness for $2\mathrm{\AA }$ each of $\mathrm{CdS}$ $(∎)$, $\mathrm{CdSe}$ $(•)$, and $\mathrm{CdTe}$ $(▴)$ deposited on the buffer. The lines are best fits to the data. As discussed in the text, the effective activation energies for cluster diffusion on the desorbing Xe buffer can be obtained with the help of these fits.

Figure 4

Low temperature PL spectra for samples of $\mathrm{CdSe}$ and $\mathrm{CdTe}$ as a function of buffer thickness after background subtraction. The excitation power was $100\mathrm{mW}$ and the excitation wavelength was $458\mathrm{nm}$ $\left(2.71\mathrm{eV}\right)$. The spectra were recorded at $3.9\mathrm{K}$, and the amount of $\mathrm{CdSe}$ or $\mathrm{CdTe}$ deposited was $2\mathrm{\AA }$ in each case. The high energy cutoff was dictated by the detector. Comparing (a) and (c) for $\mathrm{CdSe}$ shows the transition from 3D to 2D quantum confinement since the particles in (a) were compact and those in (c) were highly ramified. The redshift from (b) to (d) reflects the loss of compact particles from the mixed state of particles and rods in (b). The bulk band gaps of $\mathrm{CdSe}$ and $\mathrm{CdTe}$ are 1.73 and $1.48\mathrm{eV}$ at room temperature.

Figure 5

Integrated PL intensity vs laser power for compact $\mathrm{CdTe}$ nanoparticles. The parameter $\beta$ obtained from the fit can be used to calculate the exciton lifetime at low temperature. The sample was produced by depositing $1\mathrm{\AA }$ $\mathrm{CdTe}$ on a $10\mathrm{ML}$ Xe buffer layer. TEM analysis indicates compact particles with a peak in the size distribution at $3\mathrm{nm}$. The inset shows the PL spectrum at $100\mathrm{mW}$, with the cut off at $2.64\mathrm{eV}$ being due to the detector.

Figure 6

Temperature dependence of the peak intensity shown as a semilog plot of the integrated intensity vs the inverse temperature. The inset shows the full width at half maximum (FWHM) of the peak. These results can be used to extract the LO phonon-electron coupling coefficient and energy and also rates of phonon- and defect-assisted decay. The sample is $1\mathrm{\AA }$ $\mathrm{CdSe}$ on $6\mathrm{ML}$ Xe (compact particles with $\sim 4\mathrm{nm}$ diameter).