Morphological Expression of the Coherence and Relative Phase of Optical Inputs to the Photoelectrodeposition of Nanopatterned Se–Te Films

Abstract

Highly anisotropic and ordered nanoscale lamellar morphologies can be spontaneously generated over macroscopic areas, without the use of a photomask or any templating agents, via the photoelectrodeposition of Se–Te alloy films. To form such structures, the light source can be a single, linearly polarized light source that need not necessarily be highly coherent. In this work, the variation in the morphologies produced by this deposition process was evaluated in response to differences in the coherence and relative phase between multiple simultaneous linearly polarized illumination inputs. Specifically, the morphologies of photoelectrodeposits were evaluated when two tandem same-wavelength sources with discrete linear polarizations, both either mutually incoherent or mutually coherent (with defined phase differences), were used. Additionally, morphologies were simulated via computer modeling of the interfacial light scattering and absorption during the photoelectrochemical growth process. The morphologies that were generated using two coherent, in-phase sources were equivalent to those generated using only a single source. In contrast, the use of two coherent, out-of-phase sources produced a range of morphological patterns. For small out-of-phase addition of orthogonal polarization components, lamellar-type patterns were observed. When fully out-of-phase orthogonal sources (circular polarization) were used, an isotropic, mesh-type pattern was instead generated, similar to that observed when unpolarized illumination was utilized. In intermediate cases, anisotropic lamellar-type patterns were superimposed on the isotropic mesh-type patterns, and the relative height between the two structures scaled with the amount of out-of-phase addition of the orthogonal polarization components. Similar results were obtained when two incoherent sources were utilized. In every case, the long axis of the lamellar-type morphology component aligned parallel to the intensity-weighted average polarization orientation. The observations consistently agreed with computer simulations, indicating that the observed morphologies were fully determined by the nature of the illumination utilized during the growth process. The collective data thus indicated that the photoelectrodeposition process exhibits sensitivity toward the coherency, relative phase, and polarization orientations of all optical inputs and that this sensitivity is physically expressed in the morphology of the deposit.

Preseeded Optical Scatterers as a Template for Enhancing Order in Inorganic Phototropic Growth

“…Despite the lack of any optical anisotropy in the substrate or in the solution, the phototropically grown films clearly exhibited a preferential alignment of lamellae along the (arbitrary) direction of the incident optical polarization. The observed average 242 nm period between lamellae is in accordance with observations and expectations for inorganic phototropic growth of Se–Te from a deposition solution and growth medium with an index of refraction, n , of ∼1.3 (corresponding to the theoretical and observed trend of period = λ/2 n ). − …”

“…Phototropic growth of these semiconducting materials occurs via a single-step process that does not utilize chemical or physical templating. Highly anisotropic lamellae resulting from inorganic phototropic growth under uniform, polarized optical excitation conditions have been observed for Se–Te, − PbSe, and CdSe …”

Template-Free Synthesis of Periodic Three-Dimensional PbSe Nanostructures via Photoelectrodeposition

“…Additionally, spontaneous asymmetric light scattering and absorption during growth, followed by sustained anisotropic interfacial light collection, results in highly spatially varying local growth rates that produce the observed anisotropic growth pattern in the photoelectrodeposit. The pattern anisotropy and orientation, and feature size and pitch, are correlated with the illumination polarization and the spectral profile (wavelength distribution), respectively. − The deposit morphology is thus a physical representation of the specific photoexcitation utilized during growth, and tailoring the optical input consequently allows for the generation of complex three-dimensional (3D) nanoarchitectures.…”

Influence of Substrates on the Long-Range Order of Photoelectrodeposited Se–Te Nanostructures

“…The Se–Te alloys are semiconductors with energy-band gaps ( E g ) that are between those of Se ( E g ≈ 1.85 eV) and Te ( E g ≈ 0.33 eV), depending on the ratio of Se to Te as well as the crystallinity of the material. − Phototropic growth with consequent optically based nanoscale morphology control has also been observed for PbSe films, which suggests that the phenomenon may be general for semiconductors with short minority-carrier diffusion lengths . A model that combines full-wave optical simulations with weighted Monte Carlo mass addition accurately reproduces the average pattern period and morphology for phototropic Se–Te structures grown under multiple and complex optical inputs. ,, …”

“…7 A model that combines fullwave optical simulations with weighted Monte Carlo mass addition accurately reproduces the average pattern period and morphology for phototropic Se−Te structures grown under multiple and complex optical inputs. 2,8,9 Se−Te photoelectrodeposits exhibit phototropic growth on a variety of substrates including Au, highly oriented pyrolytic graphite, n + -Si(111), and p-Si(100). 1 Hence, the atomic-level structural properties of the substrate/film interface do not substantially influence the morphology of the resultant photoelectrodeposited phototropic Se−Te film.…”