Increased spatial randomness and disorder of nucleates in dark-phase electrodeposition lead to increased spatial order and pattern fidelity in phototropically grown Se–Te electrodeposits

The role of nucleation was investigated during phototropic growth of Se–Te. Under low levels of mass deposition (mass equivalent of −3.75 mC cm⁻² of charge passed) that produced small nucleate spacings, patterns in photoelectrochemically deposited Se–Te films converged at relatively earlier levels of mass deposition and ultimately exhibited higher pattern fidelity throughout pattern development as compared to pattern formation from larger initial nucleate spacings. Consistently, use of an applied striking potential during very early levels of mass deposition produced more spatially random dark-phase electrodeposited nucleates and led to phototropic Se–Te photoelectrodeposited films that exhibited improved pattern fidelity relative to depositions performed with no striking step. Collectively, the data indicate that increases in randomness and spatial disorder of the dispersion of the initial nucleates produces increases in the fidelity and spatial order in the resulting phototropically grown electrodeposits

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

The long-range order of anisotropic phototropic Se–Te films grown electrochemically at room temperature under uniform-intensity, polarized, incoherent, near-IR illumination has been investigated using crystalline (111)-oriented Si substrates doped degenerately with either p- or n-type dopants. Fourier-transform (FT) analysis was performed on large-area images obtained with a scanning electron microscope, and peak shapes in the FT spectra were used to determine the pattern fidelity in the deposited Se–Te films. Under nominally identical illumination conditions, phototropic films grown on p^+-Si(111) exhibited a higher degree of anisotropy and a more well-defined pattern period than phototropic films grown on n+-Si(111). Similar differences in the phototropic Se–Te deposit morphology and pattern fidelity on p^+-Si versus n^+-Si were observed when the deposition rate and current densities were controlled for by adjusting the deposition parameters and illumination conditions. The doping-related effects of the Si substrate on the pattern fidelity of the phototropic Se–Te deposits are ascribable to an electrical effect produced by the different interfacial junction energetics between Se–Te and p^+-Si versus n^+-Si that influences the dynamic behavior during phototropic growth at the Se–Te/Si interface

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

The long-range order of anisotropic phototropic Se–Te films grown electrochemically at room temperature under uniform-intensity, polarized, incoherent, near-IR illumination has been investigated using crystalline (111)-oriented Si substrates doped degenerately with either p- or n-type dopants. Fourier-transform (FT) analysis was performed on large-area images obtained with a scanning electron microscope, and peak shapes in the FT spectra were used to determine the pattern fidelity in the deposited Se–Te films. Under nominally identical illumination conditions, phototropic films grown on p^+-Si(111) exhibited a higher degree of anisotropy and a more well-defined pattern period than phototropic films grown on n+-Si(111). Similar differences in the phototropic Se–Te deposit morphology and pattern fidelity on p^+-Si versus n^+-Si were observed when the deposition rate and current densities were controlled for by adjusting the deposition parameters and illumination conditions. The doping-related effects of the Si substrate on the pattern fidelity of the phototropic Se–Te deposits are ascribable to an electrical effect produced by the different interfacial junction energetics between Se–Te and p^+-Si versus n^+-Si that influences the dynamic behavior during phototropic growth at the Se–Te/Si interface

Understanding Pattern Formation and Improving Fidelity in Phototropic Growth

Phototropic growth of Se-Te yields highly anisotropic lamellar nanostructures and is achieved photoelectrochemically from an isotropic solution of oxidized Se and Te precursors deposited onto isotropic conductive substrates under conformal illumination. In contrast to other spontaneous patterning processes, phototropic growth has no requirement for illumination source coherency and can be performed under mild conditions using low illumination power. Furthermore, as a bottom-up, solution-based synthesis, phototropic growth is scalable and demonstrates high tunability via optical input (i.e. control of wavelength and polarization). However, relative to more traditional lithographic patterning methods, phototropically grown films exhibit defective patterns which may impede their application in devices requiring high pattern fidelities. Chapter I investigates the role of the growth substrate and its effect on pattern fidelity in phototropically grown Se-Te films, quantified by peak-fitting and analysis of frequency modes in 2D Fourier transform spectra. The work function or Fermi level of the substrate was determined to be the major factor in determining pattern fidelity. Substrates that had work functions closely aligned with Se-Te (p⁺-Si and Au) demonstrated higher fidelity patterns than those that had misaligned work functions (n⁺-Si and Ti). In cases of both nominally identical illumination conditions and nominally identical growth rates, phototropically grown Se-Te films on p⁺-Si exhibited a higher degree of anisotropy and higher pattern fidelity than phototropically grown Se-Te films on n⁺-Si, attributed to energetics and charge conduction at the junction formed by the substrate and growing Se-Te film. Chapter II follows up on the analysis performed in Chapter I by investigating the role of nucleation and the earliest levels of mass addition to growth substrates in the phototropic growth of Se-Te films. In particular, the relationship between the inter-nucleate spacing of the initial dark electrodeposited material and the pattern formation pathways during the phototropic growth process is described. Conditions that produced small nucleate spacings resulted in phototropically grown films with a higher pattern fidelity and a pattern period that more strongly agreed with the theoretical trend (λ/2n). Furthermore, on substrates that generally produced low pattern fidelity films, use of an applied striking potential during the initial nucleation stage demonstrated both smaller nucleate spacings and improved pattern fidelity of resulting phototropically grown films. Finally, Chapter III investigates the effect of extrinsic (i.e. lithographically patterned) optical scattering elements on the phototropic growth process. Relative to non-templated substrates, substrates with templated ridges demonstrated higher pattern fidelities, confined pattern periods, and enforced pattern orientation. Full-wave electromagnetic modeling and Monte Carlo growth simulations of Se-Te onto simulated templated substrates resulted in simulated films demonstrating good agreement with the patterns observed experimentally. In simulation, for a given set of illumination conditions that produced a single pattern period on non-templated substrates, films grown on templated substrates were able to attain a much wider range of periods (~80% to ~160% vs. the non-templated pattern period). Additionally, the orientation of phototropically grown patterns (usually dependent on the axis polarization) were enforced to the orientation of the templates to an angular offset tolerance of up to ~40°.</p

Increased spatial randomness and disorder of nucleates in dark-phase electrodeposition lead to increased spatial order and pattern fidelity in phototropically grown Se–Te electrodeposits

The role of nucleation was investigated during phototropic growth of Se–Te. Under low levels of mass deposition (mass equivalent of −3.75 mC cm⁻² of charge passed) that produced small nucleate spacings, patterns in photoelectrochemically deposited Se–Te films converged at relatively earlier levels of mass deposition and ultimately exhibited higher pattern fidelity throughout pattern development as compared to pattern formation from larger initial nucleate spacings. Consistently, use of an applied striking potential during very early levels of mass deposition produced more spatially random dark-phase electrodeposited nucleates and led to phototropic Se–Te photoelectrodeposited films that exhibited improved pattern fidelity relative to depositions performed with no striking step. Collectively, the data indicate that increases in randomness and spatial disorder of the dispersion of the initial nucleates produces increases in the fidelity and spatial order in the resulting phototropically grown electrodeposits

Direct Observation of a Charge-Transfer State Preceding High-Yield Singlet Fission in Terrylenediimide Thin Films

Singlet exciton fission (SF) in organic chromophore assemblies results in the conversion of one singlet exciton (S₁) into two triplet excitons (T₁), provided that the overall process is exoergic, i.e., <i>E</i>(S₁) > 2<i>E</i>(T₁). We report on SF in thin polycrystalline films of two terrylene-3,4:11,12-bis­(dicarboximide) (TDI) derivatives <b>1</b> and <b>2</b>, which crystallize into two distinct π-stacked structures. Femtosecond transient absorption spectroscopy (fsTA) reveals a charge-transfer state preceding a 190% T₁ yield in films of <b>1</b>, where the π-stacked TDI molecules are rotated by 23° along an axis perpendicular to their π systems. In contrast, when the TDI molecules are slip-stacked along their N–N axes in films of <b>2</b>, fsTA shows excimer formation, followed by a 50% T₁ yield

Direct Observation of a Charge-Transfer State Preceding High-Yield Singlet Fission in Terrylenediimide Thin Films

Singlet exciton fission (SF) in organic chromophore assemblies results in the conversion of one singlet exciton (S₁) into two triplet excitons (T₁), provided that the overall process is exoergic, i.e., <i>E</i>(S₁) > 2<i>E</i>(T₁). We report on SF in thin polycrystalline films of two terrylene-3,4:11,12-bis­(dicarboximide) (TDI) derivatives <b>1</b> and <b>2</b>, which crystallize into two distinct π-stacked structures. Femtosecond transient absorption spectroscopy (fsTA) reveals a charge-transfer state preceding a 190% T₁ yield in films of <b>1</b>, where the π-stacked TDI molecules are rotated by 23° along an axis perpendicular to their π systems. In contrast, when the TDI molecules are slip-stacked along their N–N axes in films of <b>2</b>, fsTA shows excimer formation, followed by a 50% T₁ yield

Direct Observation of a Charge-Transfer State Preceding High-Yield Singlet Fission in Terrylenediimide Thin Films

Singlet exciton fission (SF) in organic chromophore assemblies results in the conversion of one singlet exciton (S₁) into two triplet excitons (T₁), provided that the overall process is exoergic, i.e., <i>E</i>(S₁) > 2<i>E</i>(T₁). We report on SF in thin polycrystalline films of two terrylene-3,4:11,12-bis­(dicarboximide) (TDI) derivatives <b>1</b> and <b>2</b>, which crystallize into two distinct π-stacked structures. Femtosecond transient absorption spectroscopy (fsTA) reveals a charge-transfer state preceding a 190% T₁ yield in films of <b>1</b>, where the π-stacked TDI molecules are rotated by 23° along an axis perpendicular to their π systems. In contrast, when the TDI molecules are slip-stacked along their N–N axes in films of <b>2</b>, fsTA shows excimer formation, followed by a 50% T₁ yield