Super-bandgap light stimulated reversible transformation and laser-driven mass transport at the surface of As2S3 chalcogenide nanolayers studied in situ

Abstract

The super-bandgap laser irradiation of the in situ prepared As-S chalcogenide films was found to cause drastic structural transformations and unexpected selective diffusion processes, leading to As enrichment on the nanolayer surface. Excitation energy dependent synchrotron radiation photoelectron spectroscopy showed complete reversibility of the molecular transformations and selective laser-driven mass transport during “laser irradiation”-“thermal annealing” cycles. Molecular modeling and density functional theory calculations performed on As-rich cage-like clusters built from basic structural units indicate that the underlying microscopic mechanism of laser induced transformations is connected with the realgar-pararealgar transition in the As-S structure. The detected changes in surface composition as well as the related local and molecular structural transformations are analyzed and a model is proposed and discussed in detail. It is suggested that the formation of a concentration gradient is a result of bond cleavage and molecular reorientation during transformations and anisotropic molecular diffusion.

Photoinduced transformations in (As1–xBix)2S3 glass observed by Raman spectroscopy

“…The possibility of existence of such structural groups (As,Bi) 2 S 1 + 4/2 and (As,Bi) 2 S 2 + 2/2 is, in particular, considered in an earlier study. 47 In general, in our opinion, a convincing unambiguous assignment of the $290 cm À1 band in the Raman spectra of the (As 1-…”

“…50,51,58,59 Another possible explanation of the appearance of intense Raman features near 180 and 230 cm À1 at the increased P exc can be related to the above mentioned transformation of As 4 S 4 clusters in the glass from realgar to pararealgar structure. 46,47 Additional contribution to the increasing maximа slightly above 180 cm À1 and near 230 cm À1 may also come from the formation of As 4 S 5 structural groups considered in detail in a recent study. 60 As can be seen from Figure 5, the drastic increase of the maxima near 180 and 230 cm À1 at P exc = 40 kW/cm 2 is accompanied by total vanishing of the feature near 290 cm À1 in the spectra of the (As 1-x Bi x ) 2 S 3 glasses related most likely, as noted above, to the vibrations of structural units with As, Bi and S atoms.…”

“…From this point of view, a suitable possibility would be to relate the ~290 cm −1 band in the spectra of the (As 1– x Bi x ) 2 S 3 glasses to vibrations of structural groups containing sulphur atoms and pnictogen atoms of two sorts (As and Bi). The possibility of existence of such structural groups (As,Bi) 2 S 1 + 4/2 and (As,Bi) 2 S 2 + 2/2 is, in particular, considered in an earlier study 47 . In general, in our opinion, a convincing unambiguous assignment of the ~290 cm −1 band in the Raman spectra of the (As 1– x Bi x ) 2 S 3 glasses is still to be found, and further studies are required for this purpose.…”

“…44,45 The increasing intensity of the feature near 230 cm À1 (in particular, at the excitation in the green spectral range) is related to a structural transformation of As-rich As 4 S 4 structural groups from realgar to pararealgar structure. 46,47 It is worth noting that earlier Raman spectra of Bidoped As 2 S 3 glasses were studied in much narrower Bi content intervals using 532 nm 48,49 and 785 nm 16 excitation. In our case, addition of bismuth (x = 0.04-0.06) to the glass composition results in an additional clearly defined peak in the Raman spectrum appearing near 290 cm À1 (Figure 1).…”

“…The As 2 S 3 Raman feature near 230 cm −1 is known to be the most sensitive to the excitation wavelength although the excitation wavelength ranges corresponding to the maximal relative intensity of this feature are somewhat different 44,45 . The increasing intensity of the feature near 230 cm −1 (in particular, at the excitation in the green spectral range) is related to a structural transformation of As‐rich As 4 S 4 structural groups from realgar to pararealgar structure 46,47 …”