Size-dependent stability of ultra-small α-/β-phase tin nanocrystals synthesized by microplasma

Abstract

Nanocrystals sometimes adopt unusual crystal structure configurations in order to maintain structural stability with increasingly large surface-to-volume ratios. The understanding of these transformations is of great scientific interest and represents an opportunity to achieve beneficial materials properties resulting from different crystal arrangements. Here, the phase transformation from α to β phases of tin (Sn) nanocrystals is investigated in nanocrystals with diameters ranging from 6.1 to 1.6 nm. Ultra-small Sn nanocrystals are achieved through our highly non-equilibrium plasma process operated at atmospheric pressures. Larger nanocrystals adopt the β-Sn tetragonal structure, while smaller nanocrystals show stability with the α-Sn diamond cubic structure. Synthesis at other conditions produce nanocrystals with mean diameters within the range 2–3 nm, which exhibit mixed phases. This work represents an important contribution to understand structural stability at the nanoscale and the possibility of achieving phases of relevance for many applications.

Tuning the Bandgap Character of Quantum‐Confined Si–Sn Alloyed Nanocrystals

“…The lowest lying excitation in the calculated absorbance spectra for the Sn NC particle with d = 1.6 nm (Figure 4a) of 2.33 eV compares well to our recent measurements where we obtained an optical gap of 2.2 eV for NCs of comparable size. [ 16 ] From the absorbance spectra of the small NC the discrete levels, suggested from the quasiband structure discussed above, is clearly visible in the absorbance spectra. Accordingly, due to the formation of quasi‐bands for the larger Sn NC ( d = 3.4 nm) the absorbance spectra (Figure 4b) tends toward bulk spectra, [ 31 ] decreasing the lowest lying excitation to around 0.98 eV.…”

“…Both the indirect bandgap for Si and the direct bandgap for α‐Sn are consistent with what our experimental findings suggest. [ 10,16 ]…”

“…For various applications such as next‐generation solar‐cells, light‐emitting diodes, and biomedical applications the optoelectronic properties of NCs in the quantum confinement regime are attractive. [ 1,8,11,14,15 ] In particular group IV semiconductors, such as tin (Sn) and silicon (Si), are very important [ 3,16 ] not only due to their electronic properties but also due to their abundance and nontoxicity, where the abundance is important for large‐scale production and the nontoxicity a necessary prerequisite for many biomedical applications. [ 3,9,12 ]…”

“…Recent experiments showed that for ultrasmall Sn nanocrystals at room temperature, contrary to bulk, the α‐phase can be stable. [ 16,20 ] For microplasma‐synthesized NCs it was found that for diameters blow 4 nm not only β‐Sn NCs but also α‐Sn NCs occur and that for ultrasmall (≈1.6 nm) NCs the α‐phase is predominant. [ 16 ] Absorbance measurements found a finite bandgap for these ultrasmall α‐Sn nanocrystals, [ 16 ] however whether the gap is indeed direct remains an open question which is difficult to answer by experiment alone.…”

“…[ 16,20 ] For microplasma‐synthesized NCs it was found that for diameters blow 4 nm not only β‐Sn NCs but also α‐Sn NCs occur and that for ultrasmall (≈1.6 nm) NCs the α‐phase is predominant. [ 16 ] Absorbance measurements found a finite bandgap for these ultrasmall α‐Sn nanocrystals, [ 16 ] however whether the gap is indeed direct remains an open question which is difficult to answer by experiment alone. Another strategy to obtain an indirect to direct bandgap transition are alloyed nanocrystals [ 21–23 ] In bulk an indirect to direct bandgap transition occurs in alloyed for y ≈ 0.25, [ 24 ] recent experiments succeeded by means of laser ablation to synthesize silicon–tin NCs in the quantum confinement regime with diameters between 1 and 5 nm [ 21,23 ] and somewhat larger silicon–tin NCs with diameters of around 15 nm were fabricated in silicon matrices.…”