Surface Chemistry in the Chemical Vapor Deposition of Electronic Materials

High-Throughput Area-Selective Spatial Atomic Layer Deposition of SiO2 with Interleaved Small Molecule Inhibitors and Integrated Back-Etch Correction for Low Defectivity

A first-of-its-kind area-selective deposition process for SiO₂  is developed consisting of film deposition with interleaved exposures to small molecule inhibitors (SMIs) and back-etch correction steps, within the same spatial atomic layer deposition (ALD) tool. The synergy of these aspects results in selective SiO₂  deposition up to ~23 nm with high selectivity and throughput, with SiO₂  growth area and ZnO nongrowth area. The selectivity is corroborated by both X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectroscopy (LEIS). The selectivity conferred by two different SMIs, ethylbutyric acid, and pivalic acid has been compared experimentally and theoretically. Density Functional Theory (DFT) calculations reveal that selective surface functionalization using both SMIs is predominantly controlled thermodynamically, while the better selectivity achieved when using trimethylacetic acid can be explained by its higher packing density compared to ethylbutyric acid. By employing the trimethylacetic acid as SMI on other starting surfaces (Ta₂ O₅ , ZrO₂ , etc.) and probing the selectivity, a broader use of carboxylic acid inhibitors for different substrates is demonstrated. It is believed that the current results highlight the subtleties in SMI properties such as size, geometry, and packing, as well as interleaved back-etch steps, which are key in developing ever more effective strategies for highly selective deposition processes.

A Critical Review on Black Phosphorus-Based Photocatalytic CO2 Reduction Application

Energy shortages and greenhouse effects are two unavoidable problems that need to be solved. Photocatalytically converting CO₂ into a series of valuable chemicals is considered to be an effective means of solving the above dilemmas. Among these photocatalysts, the utilization of black phosphorus for CO₂ photocatalytic reduction deserves a lightspot not only for its excellent catalytic activity through different reaction routes, but also on account of the great preponderance of this relatively cheap catalyst. Herein, this review offers a summary of the recent advances in synthesis, structure, properties, and application for CO₂ photocatalytic reduction. In detail, the review starts from the basic principle of CO₂ photocatalytic reduction. In the following section, the synthesis, structure, and properties, as well as CO₂ photocatalytic reduction process of black phosphorus-based photocatalyst are discussed. In addition, some possible influencing factors and reaction mechanism are also summarized. Finally, a summary and the possible future perspectives of black phosphorus-based photocatalyst for CO₂ reduction are established.

Production and Surface Modification of Cellulose Bioproducts

Petroleum-based synthetic plastics play an important role in our life. As the detrimental health and environmental effects of synthetic plastics continue to increase, the renewable, degradable and recyclable properties of cellulose make subsequent products the "preferred environmentally friendly" alternatives, with a small carbon footprint. Despite the fact that the bioplastic industry is growing rapidly with many innovative discoveries, cellulose-based bioproducts in their natural state face challenges in replacing synthetic plastics. These challenges include scalability issues, high cost of production, and most importantly, limited functionality of cellulosic materials. However, in order for cellulosic materials to be able to compete with synthetic plastics, they must possess properties adequate for the end use and meet performance expectations. In this regard, surface modification of pre-made cellulosic materials preserves the chemical profile of cellulose, its mechanical properties, and biodegradability, while diversifying its possible applications. The review covers numerous techniques for surface functionalization of materials prepared from cellulose such as plasma treatment, surface grafting (including RDRP methods), and chemical vapor and atomic layer deposition techniques. The review also highlights purposeful development of new cellulosic architectures and their utilization, with a specific focus on cellulosic hydrogels, aerogels, beads, membranes, and nanomaterials. The judicious choice of material architecture combined with a specific surface functionalization method will allow us to take full advantage of the polymer's biocompatibility and biodegradability and improve existing and target novel applications of cellulose, such as proteins and antibodies immobilization, enantiomers separation, and composites preparation.

Multimaterial Self-Aligned Nanopatterning by Simultaneous Adjacent Thin Film Deposition and Etching

Printed component sizes in electronic circuits are approaching 10 nm, but inherent variability in feature alignment during photolithography poses a fundamental barrier for continued device scaling. Deposition-based self-aligned patterning is being introduced, but nuclei defects remain an overarching problem. This work introduces low-temperature chemically self-aligned film growth via simultaneous thin film deposition and etching in adjacent regions on a nanopatterned surface. During deposition, nucleation defects are avoided in nongrowth regions because deposition reactants are locally consumed via sacrificial etching. For a range of materials and process conditions, thermodynamic modeling confirms that deposition and etching are both energetically favorable. We demonstrate nanoscale patterning of tungsten at 220 °C with simultaneous etching of TiO₂. Area selective deposition (ASD) of the sacrificial TiO₂ layer produces an orthogonal sequence for self-aligned patterning of two materials on one starting pattern, i.e., TiO₂ ASD on SiO₂ followed by W ASD on Si-H. Experiments also show capacity for self-aligned dielectric patterning via favorable deposition of AlF₃ on Al₂O₃ at 240 °C with simultaneous atomic layer etching of sacrificial ZnO. Simultaneous deposition and etching provides opportunities for low-temperature bottom-up self-aligned patterning for electronic and other nanoscale systems.

Atomically thin three-dimensional membranes of van der Waals semiconductors by wafer-scale growth

We report wafer-scale growth of atomically thin, three-dimensional (3D) van der Waals (vdW) semiconductor membranes. By controlling the growth kinetics in the near-equilibrium limit during metal-organic chemical vapor depositions of MoS₂ and WS₂ monolayer (ML) crystals, we have achieved conformal ML coverage on diverse 3D texture substrates, such as periodic arrays of nanoscale needles and trenches on quartz and SiO₂/Si substrates. The ML semiconductor properties, such as channel resistivity and photoluminescence, are verified to be seamlessly uniform over the 3D textures and are scalable to wafer scale. In addition, we demonstrated that these 3D films can be easily delaminated from the growth substrates to form suspended 3D semiconductor membranes. Our work suggests that vdW ML semiconductor films can be useful platforms for patchable membrane electronics with atomic precision, yet large areas, on arbitrary substrates.

Nano-Sized Inorganic Energy-Materials by the Low-Temperature Molecular Precursor Approach

The low-temperature synthesis of inorganic materials and their interfaces at the atomic and molecular level provides numerous opportunities for the design and improvement of inorganic materials in heterogeneous catalysis for sustainable chemical energy conversion or other energy-saving areas. Using suitable molecular precursors for functional inorganic nanomaterial synthesis allows for facile control over uniform particle size distribution, stoichiometry, and leads to desired chemical and physical properties. This Minireview outlines some advantages of the molecular precursor approach in light of selected recent developments of molecule-to-nanomaterials synthesis for renewable energy applications, relevant for the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water-splitting.

Bulked sample analysis in genetics, genomics and crop improvement

Biological assay has been based on analysis of all individuals collected from sample populations. Bulked sample analysis (BSA), which works with selected and pooled individuals, has been extensively used in gene mapping through bulked segregant analysis with biparental populations, mapping by sequencing with major gene mutants and pooled genomewide association study using extreme variants. Compared to conventional entire population analysis, BSA significantly reduces the scale and cost by simplifying the procedure. The bulks can be built by selection of extremes or representative samples from any populations and all types of segregants and variants that represent wide ranges of phenotypic variation for the target trait. Methods and procedures for sampling, bulking and multiplexing are described. The samples can be analysed using individual markers, microarrays and high-throughput sequencing at all levels of DNA, RNA and protein. The power of BSA is affected by population size, selection of extreme individuals, sequencing strategies, genetic architecture of the trait and marker density. BSA will facilitate plant breeding through development of diagnostic and constitutive markers, agronomic genomics, marker-assisted selection and selective phenotyping. Applications of BSA in genetics, genomics and crop improvement are discussed with their future perspectives.

Impact of surface chemistry

The applications of molecular surface chemistry in heterogeneous catalyst technology, semiconductor-based technology, medical technology, anticorrosion and lubricant technology, and nanotechnology are highlighted in this perspective. The evolution of surface chemistry at the molecular level is reviewed, and the key roles of surface instrumentation developments for in situ studies of the gas-solid, liquid-solid, and solid-solid interfaces under reaction conditions are emphasized.

Profile evolution for conformal atomic layer deposition over nanotopography

The self-limiting reactions which distinguish atomic layer deposition (ALD) provide ultrathin film deposition with superb conformality over the most challenging topography. This work addresses how the shapes (i.e., surface profiles) of nanostructures are modified by the conformality of ALD. As a nanostructure template, we employ a highly scalloped surface formed during the first anodization of the porous anodic alumina (PAA) process, followed by removal of the alumina to expose a scalloped Al surface. SEM and AFM reveal evolution of surface profiles that change with ALD layer thickness, influenced by the way ALD conformality decorates the underlying topography. The evolution of surface profiles is modeled using a simple geometric 3D extrusion model, which replicates the measured complex surface topography. Excellent agreement is obtained between experimental data and the results from this model, suggesting that for this ALD system conformality is very high even on highly structured, sharp features of the initial template surface. Through modeling and experimentation, the benefits of ALD to manipulate complex surface topographies are recognized and will play an important role in the design and nanofabrication of next generation devices with increasingly high aspect ratios as well as nanoscale features.

A conformal nano-adhesive via initiated chemical vapor deposition for microfluidic devices

A novel high-strength nano-adhesive is demonstrated for fabricating nano- and microfluidic devices. While the traditional plasma sealing methods are specific for sealing glass to poly(dimethylsiloxane) (PDMS), the new method is compatible with a wide variety of polymeric and inorganic materials, including flexible substrates. Additionally, the traditional method requires that sealing occur within minutes after the plasma treatment. In contrast, the individual parts treated with the nano-adhesive could be aged for at least three months prior to joining with no measurable deterioration of post-cure adhesive strength. The nano-adhesive is comprised of a complementary pair of polymeric nanolayers. An epoxy-containing polymer, poly(glycidyl methacrylate) (PGMA) was grown via initiated chemical vapor deposition (iCVD) on the substrate containing the channels. A plasma polymerized polyallylamine (PAAm) layer was grown on the opposing flat surface. Both CVD monomers are commercially available. The PGMA nano-adhesive layer displayed conformal coverage over the channels and was firmly tethered to the substrate. Contacting the complementary PGMA and PAAm surfaces, followed by curing at 70 degrees C, resulted in nano- and micro-channel structures. The formation of the covalent tethers between the complementary surfaces produces no gaseous by-products which would need to outgas. The nano-adhesive layers did not flow significantly as a result of curing, allowing the cross-sectional profile of the channel to be maintained. This enabled fabrication of channels with widths as small as 200 nm. Seals able to withstand > 50 psia were fabricated employing many types of substrates, including silicon wafer, glass, quartz, PDMS, polystyrene petri dishes, poly(ethylene terephthalate) (PET), polycarbonate (PC), and poly(tetrafluoro ethylene) (PTFE).

Conformal coverage of poly(3,4-ethylenedioxythiophene) films with tunable nanoporosity via oxidative chemical vapor deposition

Novel nanoporous poly(3,4-ethylenedioxythiophene) (PEDOT) films with basalt-like surface morphology are successfully obtained via a one-step, vapor phase process of oxidative chemical vapor deposition (oCVD) by introducing a new oxidant, CuCl(2). The substrate temperature of the oCVD process is a crucial process parameter for controlling electrical conductivity and conjugation length. Moreover, the surface morphology is also systemically tunable through variations in substrate temperature, a unique advantage of the oCVD process. By increasing the substrate temperature, the surface morphology becomes more porous, with the textured structure on the nanometer scale. The size of nanopores and fibrils appears uniformly over 25 mm x 25 mm areas on the Si wafer substrates. Conformal coverage of PEDOT films grown with the CuCl(2) oxidant (C-PEDOT) is observed on both standard trench structures with high aspect ratio and fragile surfaces with complex topology, such as paper, results which are extremely difficult to achieve with liquid phase based processes. The tunable nanoporosity and its conformal coverage on various complex geometries are highly desirable for many device applications requiring controlled, high interfacial area, such as supercapacitors, Li ion battery electrodes, and sensors. For example, a highly hydrophilic surface with the static water contact angle down to less than 10 degrees is obtained solely by changing surface morphology. By applying fluorinated polymer film onto the nanoporous C-PEDOT via initiative chemical vapor deposition (iCVD), the C-PEDOT surface also shows the contact angle higher than 150 degrees . The hierarchical porous structure of fluorinated polymer coated C-PEDOT on a paper mat shows superhydrophobicity and oil repellency.

Molecular dynamics methodology to investigate steady-state heterogeneous crystal growth

In this paper a new molecular dynamics simulation methodology to investigate steady-state heterogeneous crystal growth from a supercooled liquid is presented. The method is tested on pure component systems such as Lennard-Jonesium and water/ice, as well as multicomponent systems such as methane hydrate crystals. The setup uses periodicity in all three directions and two interfaces; at one interface, crystallization occurs, while at the other, melting is enforced by locally heating the crystal only near that interface. Steady-state conditions are achieved when the crystal is melted at the same rate as the growth occurs. A self-adaptive scheme that automatically modifies the rate of melting to match the rate of growth, crucial for establishing steady-state conditions, is described. In contrast with the recently developed method of Razul et al. [Mol. Phys. 103, 1929 (2005)], where the rates of growth (melting) were constant and the temperatures determined, the present approach fixes the supercooling temperature at the growing interface and identifies the corresponding steady-state crystal growth rate that corresponds to the thermodynamic force provided. The static properties of the interface (e.g., the interfacial widths) and the kinetics of the crystal growth are found to reproduce well previous findings. The importance of establishing steady-state conditions in such investigations is also briefly discussed.

From "parasitic" association reactions toward the stoichiometry controlled gas phase synthesis of nanoparticles: a theoretically driven challenge for experimentalists

In the present record a model for the gas-phase reactions during the chemical vapor deposition (CVD) processes of group 13-15 materials is presented, based on the results of extensive quantum-chemical modeling. Thermodynamic criteria have been introduced to evaluate the importance of a range of association reactions. For the organometallic and hydride derivatives, association processes are found to be favorable both thermodynamically and kinetically. Formation of high mass association products takes place under CVD conditions, including laser-assisted CVD. Structural and thermodynamic properties of the most important ring and cluster intermediates have been predicted. The stoichiometry-controlled synthesis of the 13-15 ternary alloys and nanoparticles using cluster compounds as single-source precursors is predicted to be viable. The association pathway described may be generalized to the CVD reactions of many binary materials (12-16, 13-16, 13-15, 14-15, 14-16).

Protein microarrays: challenges and promises

Genomics and proteomics are playing increasingly important roles as discovery tools in basic biological sciences and as diagnostic and rational therapeutic aids in the clinical arena. In recent years, high-density arrays of specified DNA sequences have gained popularity. Protein microarrays are at the forefront of this biochip revolution and promise the parallel examination of large numbers of proteins. These miniaturized arrays are currently being developed to facilitate high analytical resolution, detection sensitivity and sample throughput. Many challenges are presented by proteome scale manipulation of proteins, as there is currently no methodological equivalent to the gene chip for comparative proteomics.