Solution-Processed Zinc Phosphide (α-Zn₃P₂) Colloidal Semiconducting Nanocrystals for Thin Film Photovoltaic Applications

Zinc phosphide (Zn₃P₂) is a promising earth-abundant material for thin film photovoltaic applications, due to strong optical absorption and near ideal band gap. In this work, crystalline zinc phosphide nanoparticles are synthesized using dimethylzinc and tri-<i>n</i>-octylphosphine as precursors. Transmission electron microscopy and X-ray diffraction data show that these nanoparticles have an average diameter of ∼8 nm and adopt the crystalline structure of tetragonal α-Zn₃P₂. The optical band gap is found to increase by 0.5 eV relative to bulk Zn₃P₂, while there is an asymmetric shift in the conduction and valence band levels. Utilizing layer-by-layer deposition of Zn₃P₂ nanoparticle films, heterojunction devices consisting of ITO/ZnO/Zn₃P₂/MoO₃/Ag are fabricated and tested for photovoltaic performance. The devices are found to exhibit excellent rectification behavior (rectification ratio of 600) and strong photosensitivity (on/off ratio of ∼10²). X-ray photoelectron spectroscopy and ultraviolet photoemission spectroscopy analyses reveal the presence of a thin 1.5 nm phosphorus shell passivating the surface of the Zn₃P₂ nanoparticles. This shell is believed to form during the nanoparticle synthesis

Preferential Alignment of Incommensurate Block Copolymer Dot Arrays Forming Moiré Superstructures

Block copolymer (BCP) self-assembly is of great interest as a cost-effective method for large-scale, high-resolution nanopattern fabrication. Directed self-assembly can induce long-range order and registration, reduce defect density, and enable access to patterns of higher complexity. Here we demonstrate preferential orientation of two incommensurate BCP dot arrays. A bottom layer of hexagonal silica dots is prepared <i>via</i> typical self-assembly from a PS-<i>b</i>-PDMS block copolymer. Self-assembly of a second, or top, layer of a different PS-<i>b</i>-PDMS block copolymer that forms a hexagonal dot pattern with different periodicity results in a predictable moiré superstructure. Four distinct moiré superstructures were demonstrated through a combination of different BCPs and different order of annealing. The registration force of the bottom layer of hexagonal dots is sufficient to direct the self-assembly of the top layer to adopt a preferred relative angle of rotation. Large-area helium ion microscopy imaging enabled quantification of the distributions of relative rotations between the two lattices in the moiré superstructures, yielding statistically meaningful results for each combination. It was also found that if the bottom layer dots were too large, the resulting moiré pattern was lost. A small reduction in the bottom layer dot size, however, resulted in large-area moiré superstructures, suggesting a specific size regime where interlayer registration forces can induce long-range preferential alignment of incommensurate BCP dot arrays

Nanopatterning via Solvent Vapor Annealing of Block Copolymer Thin Films

The self-assembly of block copolymers to generate nanopatterns is of great interest as an inexpensive approach to sub-20 nm lithography. Compared to thermal annealing, solvent vapor annealing has several intriguing advantages with respect to the annealing of thin films of block copolymers, particularly for polymers with high interaction parameters, χ, and high molecular weights. In this methods paper, we describe a controlled solvent vapor flow annealing system with integrated in situ microscopy and laser reflectometry, as well as a feedback loop that automatically controls the solvent vapor flow rate, based upon real-time calculations of the difference between thickness set point and the observed film thickness. The feedback loop enables precise control of swelling and deswelling of the polymer thin film, the degree of swelling at the dwell period, and preprogrammed complex multistep annealing profiles. The in situ microscope provides critical insight into the morphological evolution of the block copolymer thin films over a broad area of the sample, revealing information about terraced phases, on the scale of tens and hundreds of micrometers, during the annealing process. This device could be a powerful tool for understanding and optimizing solvent annealing by providing multiple sources of in situ information, at both the micro- and nanoscales

Vapor-Phase Nanopatterning of Aminosilanes with Electron Beam Lithography: Understanding and Minimizing Background Functionalization

Electron beam lithography (EBL) is a highly precise, serial method for patterning surfaces. Positive tone EBL resists enable patterned exposure of the underlying surface, which can be subsequently functionalized for the application of interest. In the case of widely used native oxide-capped silicon surfaces, coupling an activated silane with electron beam lithography would enable nanoscale chemical patterning of the exposed regions. Amino­alk­oxy­silanes are extremely useful due to their reactive amino functionality but have seen little attention for nanopatterning silicon surfaces with an EBL resist due to background contamination. In this work, we investigated three commercial positive tone EBL resists, PMMA (950k and 495k) and ZEP520A (57k), as templates for vapor-phase patterning of two commonly used amino­alk­oxy­silanes, 3-amino­propyl­tri­meth­oxy­silane (APTMS) and 3-amino­propyl­di­iso­propyl­eth­oxy­silane (APDIPES). The PMMA resists were susceptible to significant background reaction within unpatterned areas, a problem that was particularly acute with APTMS. On the other hand, with both APTMS and APDIPES exposure, unpatterned regions of silicon covered by the ZEP520A resist emerged pristine, as shown both with SEM images of the surfaces of the underlying silicon and through the lack of electrostatically driven binding of negatively charged gold nanoparticles. The ZEP520A resist allowed for the highly selective deposition of these alk­oxy­amino­silanes in the exposed areas, leaving the unpatterned areas clean, a claim also supported by contact angle measurements with four probe liquids and X-ray photoelectron spectroscopy (XPS). We investigated the mechanistic reasons for the stark contrast between the PMMA resists and ZEP520A, and it was found that the efficacy of resist removal appeared to be the critical factor in reducing the background functionalization. Differences in the molecular weight of the PMMA resists and the resulting influence on APTMS diffusion through the resist films are unlikely to have a significant impact. Area-selective nanopatterning of 15 nm gold nanoparticles using the ZEP520A resist was demonstrated, with no observable background conjugation noted in the unexposed areas on the silicon surface by SEM

Polymers, Plasmons, and Patterns: Mechanism of Plasmon-Induced Hydrosilylation on Silicon

Directed assembly for nanopatterning on semiconductor surfaces is of interest as a cost-effective approach for lithography on silicon, which is complementary to photolithography. In this work, self-assembly of block copolymers is used to produce nanoscale hexagonal arrays of gold hemispheroids, which are then incorporated into an optically transparent, flexible PDMS stamp. These “plasmonic stamps” can then be used to drive hydrosilylation of alkenes and alkynes on hydride-terminated silicon surfaces upon illumination with low-intensity green light [which corresponds with the absorption of the localized surface plasmon resonance (LSPR) of the gold nanostructures]. The resulting hexagonal arrays of nanoscale alkyl or alkenyl patches mirror the spacing of gold nanoparticles in the parent plasmonic stamp. Close examination of the hydrosilylated patches reveals that they are not continuous across the 20–30 nm diameter patches but instead display an annular motif, which closely resembles the plasmonic electric field (E-field) distribution of the gold hemispheroids embedded within the stamp. The localized surface plasmon appears to drive the hydrosilylation reaction on the silicon surface via formation of electron–hole pairs within the silicon, or injection of hot holes. The yield of hydrosilylation is, however, strongly influenced by the doping of the silicon, and the distance between the plasmonic stamp and the silicon surface. A more nuanced mechanism is thus proposed, involving band bending at the metal–insulator–semiconductor junction, where plasmonically injected/generated holes are swept toward the surface. The accumulation of holes at the silicon surface is the key element of the mechanism, as this step is followed by nucleophilic attack of the alkene or alkyne, to produce the silicon–carbon bond

Sequential Nanopatterned Block Copolymer Self-Assembly on Surfaces

Bottom-up self-assembly of high-density block-copolymer nanopatterns is of significant interest for a range of technologies, including memory storage and low-cost lithography for on-chip applications. The intrinsic or native spacing of a given block copolymer is dependent upon its size (<i>N</i>, degree of polymerization), composition, and the conditions of self-assembly. Polystyrene-<i>block</i>-polydimethylsiloxane (PS<i>-b</i>-PDMS) block copolymers, which are well-established for the production of strongly segregated single-layer hexagonal nanopatterns of silica dots, can be layered sequentially to produce density-doubled and -tripled nanopatterns. The center-to-center spacing and diameter of the resulting silica dots are critical with respect to the resulting double- and triple-layer assemblies because dot overlap reduces the quality of the resulting pattern. The addition of polystyrene (PS) homopolymer to PS<i>-b</i>-PDMS reduces the size of the resulting silica dots but leads to increased disorder at higher concentrations. The quality of these density-multiplied patterns can be calculated and predicted using parameters easily derived from SEM micrographs of corresponding single and multilayer patterns; simple geometric considerations underlie the degree of overlap of dots and layer-to-layer registration, two important factors for regular ordered patterns, and clearly defined dot borders. Because the higher-molecular-weight block copolymers tend to yield more regular patterns than smaller block copolymers, as defined by order and dot circularity, this sequential patterning approach may provide a route toward harnessing these materials, thus surpassing their native feature density

Nanoscale Plasmonic Stamp Lithography on Silicon

Nanoscale lithography on silicon is of interest for applications ranging from computer chip design to tissue interfacing. Block copolymer-based self-assembly, also called directed self-assembly (DSA) within the semiconductor industry, can produce a variety of complex nanopatterns on silicon, but these polymeric films typically require transformation into functional materials. Here we demonstrate how gold nanopatterns, produced <i>via</i> block copolymer self-assembly, can be incorporated into an optically transparent flexible PDMS stamp, termed a plasmonic stamp, and used to directly functionalize silicon surfaces on a sub-100 nm scale. We propose that the high intensity electric fields that result from the localized surface plasmons of the gold nanoparticles in the plasmonic stamps upon illumination with low intensity green light, lead to generation of electron–hole pairs in the silicon that drive spatially localized hydrosilylation. This approach demonstrates how localized surface plasmons can be used to enable functionalization of technologically relevant surfaces with nanoscale control

UV-Initiated Si–S, Si–Se, and Si–Te Bond Formation on Si(111): Coverage, Mechanism, and Electronics

Diaryl and dialkyl chalcogenide molecules serve as convenient precursors to silicon–chalcogenide bonds, Si–E–R groups, on silicon surfaces, where E = S, Se, and Te. The 254 nm light, coupled with gentle heating to melt and liquefy the chalcogenide precursors for 15 min, enables formation of the resulting silicon–chalcogenide bonds. R groups analyzed comprise a long alkyl chain, octadecyl, and a phenyl group. Quantification of substitution levels of the silicon-hydride on the starting Si(111)–H surface by an organochalcogen was determined by XPS, using the chalcogenide linker atom as the atomic label, where average substitution levels of ∼15% were found for all Si–E–Ph groups. These measured substitution levels were found to agree with 2-dimensional stochastic simulations assuming kinetically irreversible silicon–chalcogen bond formation. Due to the small bond angle about the chalcogen atom, the phenyl rings in the case of Si–E–Ph effectively block otherwise reactive Si–H bonds, leading to the observed lower substitution levels. The linear aliphatic dialkyl disulfide version, Si–S–<i>n</i>-octadecyl, is less limited by steric blocking of surface Si–H groups as is the case with a phenyl group and has a much higher substitution level of ∼29%. The series, Si–S–Ph, Si–Se–Ph, and Si–Te–Ph, was prepared to determine the effect of chalcogenide substitution on the electronics of the silicon, including surface dipoles and work function. The electronics did not change significantly from the starting Si–H surface, which may be due to the low level of substitution that is believed to be caused by steric blocking by the phenyl groups, as well as the relatively similar electronegativities of these elements relative to silicon

UV-Initiated Si–S, Si–Se, and Si–Te Bond Formation on Si(111): Coverage, Mechanism, and Electronics

Diaryl and dialkyl chalcogenide molecules serve as convenient precursors to silicon–chalcogenide bonds, Si–E–R groups, on silicon surfaces, where E = S, Se, and Te. The 254 nm light, coupled with gentle heating to melt and liquefy the chalcogenide precursors for 15 min, enables formation of the resulting silicon–chalcogenide bonds. R groups analyzed comprise a long alkyl chain, octadecyl, and a phenyl group. Quantification of substitution levels of the silicon-hydride on the starting Si(111)–H surface by an organochalcogen was determined by XPS, using the chalcogenide linker atom as the atomic label, where average substitution levels of ∼15% were found for all Si–E–Ph groups. These measured substitution levels were found to agree with 2-dimensional stochastic simulations assuming kinetically irreversible silicon–chalcogen bond formation. Due to the small bond angle about the chalcogen atom, the phenyl rings in the case of Si–E–Ph effectively block otherwise reactive Si–H bonds, leading to the observed lower substitution levels. The linear aliphatic dialkyl disulfide version, Si–S–<i>n</i>-octadecyl, is less limited by steric blocking of surface Si–H groups as is the case with a phenyl group and has a much higher substitution level of ∼29%. The series, Si–S–Ph, Si–Se–Ph, and Si–Te–Ph, was prepared to determine the effect of chalcogenide substitution on the electronics of the silicon, including surface dipoles and work function. The electronics did not change significantly from the starting Si–H surface, which may be due to the low level of substitution that is believed to be caused by steric blocking by the phenyl groups, as well as the relatively similar electronegativities of these elements relative to silicon

Phase-Pure Crystalline Zinc Phosphide Nanoparticles: Synthetic Approaches and Characterization

Zinc phosphide may have potential for photovoltaic applications due to its high absorptivity of visible light and the earth abundance of its constituent elements. Two different solution-phase synthetic strategies for phase-pure and crystalline Zn₃P₂ nanoparticles (∼3–15 nm) are described here using dimethylzinc and vary with phosphorus source. Use of tri-<i>n</i>-octylphosphine (TOP) with ZnMe₂ takes place at high temperatures (∼350 °C) and appears to proceed via rapid <i>in situ</i> reduction to Zn(0), followed by subsequent reaction with TOP over a period of several hours to produce Zn₃P₂ nanoparticles. Some degree of control over size was obtained through variance of the TOP concentration in solution; the average size of the particles decreases with increasing TOP concentration. With the more reactive phosphine, P­(SiMe₃)₃, lower temperatures, ∼150 °C, and shorter reaction times (1 h) are required. When P­(SiMe₃)₃ is used, the reaction mechanism most likely proceeds via phosphido-bridged dimeric Zn­(II) intermediates, and not metallic zinc species, as is the case with TOP. In all cases, the nanoparticles were characterized by a combination of X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and solution and solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) analyses. Surface investigation through a combination of MAS ³¹P NMR and XPS analyses suggests that the particles synthesized with TOP at 350 °C possess a core–shell structure consisting of a crystalline Zn₃P₂ core and an amorphous P(0)-rich shell. Conversely, the ligand and phosphorus sources are decoupled in the P­(SiMe₃)₃ synthesis, resulting in significantly reduced P(0) formation