Multinary Tetrahedrite (Cu12-x-yMxNySb4S13) Nanoparticles: Tailoring Thermal and Optical Properties with Copper-Site Dopants.

Tetrahedrite (Cu12Sb4S13) is an earth-abundant and nontoxic compound with prospective applications in green energy technologies such as thermoelectric waste heat recycling or photovoltaic power generation. A facile, one-pot solution-phase modified polyol method has been developed that produces high-purity nanoscale tetrahedrite products with exceptional stoichiometric and phase control. This modified polyol method is used here to produce phase-pure quaternary and quintenary tetrahedrite nanoparticles doped on the Cu-site with Zn, Fe, Ni, Mn, or Co. This is the first time that Cu-site codoped quintenary tetrahedrite and Mn-doped quaternary tetrahedrite have been produced by a solution-phase method. X-ray diffraction shows phase-pure tetrahedrite, while scanning and transmission electron microscopy show the size and morphology of the nanomaterials. Energy dispersive X-ray spectroscopy confirms nanoparticles have near-stoichiometric elemental compositions. Thermal stability of quintenary codoped tetrahedrite material is analyzed using thermogravimetric analysis, finding that codoping with Mn, Fe, Ni, and Zn increased thermal stability while codoping with cobalt decreased thermal stability. This is the first systematic study of the optical properties of quaternary and quintenary tetrahedrite nanoparticles doped on the Cu-site. Visible-NIR diffuse reflectance spectroscopy reveals that the quaternary and quintenary tetrahedrite nanoparticles have direct optical band gaps ranging from 1.88 to 2.04 eV. Data from thermal and optical characterization support that codoped tetrahedrite nanoparticles are composed of quintenary grains. This research seeks to enhance understanding of the material properties of tetrahedrite, leading to the optimization of sustainable, nontoxic, and high-performance photovoltaic and thermoelectric materials.

Nanomaterials Research at a Primarily Undergraduate Institution: Transforming Nanorods, Undergraduate Research Communities, and Infrastructure.

Undergraduate research transforms student's conceptions of themselves as scientists and encourages participation and retention in science, technology, engineering, and mathematics (STEM) fields. Many barriers exist to carrying out scientifically impactful undergraduate research in nanomaterials at primarily undergraduate institutions (PUIs). Here, we share several practices and design principles that demonstrate pathways to overcome these barriers. Design of modular research projects with low entry barriers is essential. Postsynthetic transformation of nanoparticles is a field that enables such design and has been used successfully to advance nanoscience research while being achievable within undergraduate laboratories. Relatively large, inclusive research communities can be supported through the creation of opportunities with peer- and near-peer mentoring. We also share emerging strategies for enabling routine undergraduate access to transmission electron microscopy, which is one of the most mainstream characterization techniques in nanoscience yet is frequently absent from the infrastructure at undergraduate-focused institutions.

Temperature-Dependent Selection of Reaction Pathways, Reactive Species, and Products during Postsynthetic Selenization of Copper Sulfide Nanoparticles.

Rational design of elaborate, multicomponent nanomaterials is important for the development of many technologies such as optoelectronic devices, photocatalysts, and ion batteries. Combination of metal chalcogenides with different anions, such as in CdS/CdSe structures, is particularly effective for creating heterojunctions with valence band offsets. Seeded growth, often coupled with cation exchange, is commonly used to create various core/shell, dot-in-rod, or multipod geometries. To augment this library of multichalcogenide structures with new geometries, we have developed a method for postsynthetic transformation of copper sulfide nanorods into several different classes of nanoheterostructures containing both copper sulfide and copper selenide. Two distinct temperature-dependent pathways allow us to select from several outcomes-rectangular, faceted Cu2-xS/Cu2-xSe core/shell structures, nanorhombuses with a Cu2-xS core, and triangular deposits of Cu2-xSe or Cu2-x(S,Se) solid solutions. These different outcomes arise due to the evolution of the molecular components in solution. At lower temperatures, slow Cu2-xS dissolution leads to concerted morphology change and Cu2-xSe deposition, while Se-anion exchange dominates at higher temperatures. We present detailed characterization of these Cu2-xS-Cu2-xSe nanoheterostructures by transmission electron microscopy (TEM), powder X-ray diffraction, energy-dispersive X-ray spectroscopy, and scanning TEM-energy-dispersive spectroscopy. Furthermore, we correlate the selenium species present in solution with the roles they play in the temperature dependence of nanoheterostructure formation by comparing the outcomes of the established reaction conditions to use of didecyl diselenide as a transformation precursor.

Self-assembly of isomeric monofunctionalized thiophenes.

Controlling the self-assembly of thiophene-containing molecules and polymers requires a strong fundamental understanding of the relationship between molecular features and structure-directing forces. Here, the effects of ring-substitution position on the two-dimensional self-assembly of monosubstituted thiophenes at the phenyloctane/HOPG interface are studied using scanning tunneling microscopy (STM). The influence of π···π-stacking, hydrogen-bonding, and alkyl-chain interactions are explored computationally. Alteration of the amide attachment point from the 2- to the 3-position induces transformation from head-to-tail packing to head-to-head packing. This may be attributed to canceling of lateral dipoles.

Heterostructures of Cu2-xS/Cu2-xTe plasmonic semiconductors: disappearing and reappearing LSPR with anion exchange.

Localized surface plasmon resonance (LSPR) of Cu2-xS nanorods is quenched during the initial Cu2-xS/Cu2-xTe core/shell stage of anion exchange then returns as Cu2-xTe progresses into the nanorod. Phase change within the core accounts for this behaviour illustrating the complexity emergent from anion exchange.

The Primarily Undergraduate Nanomaterials Cooperative: A New Model for Supporting Collaborative Research at Small Institutions on a National Scale.

The Primarily Undergraduate Nanomaterials Cooperative (PUNC) is an organization for research-active faculty studying nanomaterials at Primarily Undergraduate Institutions (PUIs), where undergraduate teaching and research go hand-in-hand. In this perspective, we outline the differences in maintaining an active research group at a PUI compared to an R1 institution. We also discuss the work of PUNC, which focuses on community building, instrument sharing, and facilitating new collaborations. Currently consisting of 37 members from across the United States, PUNC has created an online community consisting of its Web site (nanocooperative.org), a weekly online summer group meeting program for faculty and students, and a Discord server for informal conversations. Additionally, in-person symposia at ACS conferences and PUNC-specific conferences are planned for the future. It is our hope that in the years to come PUNC will be seen as a model organization for community building and research support at primarily undergraduate institutions.

Contrasting two- and three-dimensional crystal properties of isomeric dialkyl phthalates.

Competitive adsorption occurs whenever a solution containing multiple components is in contact with a surface, and the relative adsorption strengths affect both the properties of the interface and the solution. To investigate this phenomenon, the factors influencing relative adsorption strengths of a series of isomeric dialkyl phthalates were determined from their competitive adsorption behavior and interpreted in the context of the thermodynamic properties of the bulk crystals and the structural features of the monolayers. The order of stabilities of the two-dimensional crystals paralleled that of the three-dimensional crystals, as determined by the trends in melting point, enthalpy of melting, and solubility. The magnitude of the stability differences, however, could only be understood through examination of the structures of the monolayers. Thus, the important process of adsorption of solutes from solution must be viewed as a microscopic phenomenon; adsorption strengths are influenced not only by molecular and macroscopic properties but also by the distinct assembly mode of the adsorbed layer.

Spatial and temporal control over adsorption from multicomponent solutions.

The composition of physisorbed monolayers formed from multicomponent solutions varies with time and selection between two phases is possible by controlled desorption of one phase and the kinetically favored adsorption of another.

Stabilization of plasmon resonance in Cu2-xS semiconductor nanoparticles.

Controllable copper vacancy concentrations in copper chalcogenides are essential to any application that requires constant NIR absorption behavior, including cancer phototherapy and photovoltaics. Doping levels, however, can change spontaneously and with oxygen exposure. Treatment of copper sulphide nanoparticles with tetrathiomolybdate is shown here to stabilize vacancy-induced plasmon bands.

Molecular packing and symmetry of two-dimensional crystals.

Periodic arrangements on surfaces resulting from monolayer formation are critical in determining the electronic structure of thin films, the adhesion of surface coatings, the properties of lubricants, and the polymorphic form of heteronucleated crystals. Unlike substrate-directed chemisorption, the process of physisorption is highly responsive to molecular structure and stands out as a controllable method of creating variable surface patterns with periodicities on the low end of the nanoscale. Despite decades of study focused upon such ordered structures, the principles guiding the formation of these two-dimensional crystals have been obscured by the lack of a systematic and critical compilation. Thus, prediction of two-dimensional structure based upon the composition of the individual building blocks remains in its infancy. Here we demonstrate through the compilation and analysis of a database of two-dimensional structures that molecular-scale patterns are dictated by the same factors that determine bulk crystal structure, but these factors give rise to different preferred packing symmetries. In marked contrast to three-dimensional systems, achiral molecules in two-dimensional crystals are likely to adopt chiral structures, and racemic mixtures are expected to produce enantiopure domains. The determination of plane group frequencies allowed experimental verification of Kitaigorodskii's 50-year old theory of close packing as applied to two-dimensional tiling. This fundamental comparison between bulk crystals and physisorbed monolayers provides new tools and directions for future exploration in the engineering of surfaces with prescribed two-dimensional patterns.

Crystal structure, dissolution, and deposition of a 5 nm functionalized metal-organic great rhombicuboctahedron.

A metal-organic great rhombicuboctahedron (termed MOP-18) was functionalized with extended alkyl chains and crystallized. It has 12 rigid square-shaped Cu2(CO2)4 paddle-wheel building units and 24 5-dodecoxybenzene-1,3-dicarboxylate links. MOP-18 is soluble in a variety of organic solvents, such as tetrahydrofuran, chloroform, and toluene, which enabled its assembly on graphite substrate and its observation by scanning tunneling microscopy.

Large-periodicity two-dimensional crystals by cocrystallization.

Patterning surfaces with features on the low end of the nanoscale can efficiently be accomplished with physisorbed monolayers. Here, cocrystallization is revealed as a powerful approach toward dramatically increasing the periodicity of surface features and expanding the length scale on which these patterns can form. By variation of the ratio of adsorbates in solution, surface composition can be controlled such that features on the length scale of several molecules are obtained, offering a facile approach to surface nanopatterning.

Conformational pseudopolymorphism and orientational disorder in two-dimensional alkyl carbamate crystals.

The structures of self-assembled physisorbed monolayers of alkyl carbamates were examined with atomic detail by scanning tunneling microscopy at the liquid-solid interface. Through systematic variation of molecular structure, the factors determining the two-dimensional crystal packing and dynamics of alkyl carbamate monolayers were isolated. Two different conformational pseudopolymorphs on the surface were observed and their order of stability was varied by changing the length of the alkyl groups. The relative size of the two alkyl groups in a molecule affected the frequency of orientational flipping within a column, which in turn, exerts an influence on the relative orientation of the two-dimensional crystalline domains. These phenomena were explained on the basis of the preferred hydrogen-bonding geometry of the carbamate functional group and the different degree of van der Waals interaction for each form.

Structure of and competitive adsorption in alkyl dicarbamate two-dimensional crystals.

The potential for relatively minor structural changes to dramatically impact materials properties is one of the primary obstacles to achieving the rational design of functional materials. For example, having an odd versus an even number of carbons between functional groups in polymers can cause large variation in melting point and mechanical properties. This odd-even effect is especially pronounced in hydrogen-bonded polymers and oligomers. To shed light on the structural basis of this phenomenon, physisorbed monolayers and single crystals of alkyl dicarbamates were investigated by scanning tunneling microscopy and X-ray diffraction, respectively. The related two- and three-dimensional crystal structures both demonstrated a clear odd-even effect in packing geometry. The differing accommodation of intermolecular interactions between odd and even packing motifs was directly related to the melting point trends and further dissected through computation. In addition, these oligomers displayed unusual competitive adsorption behavior; the relative preference for adsorption of a smaller species from a binary solution was increased compared to alkanes. These results were explained in the context of hydrogen bond density effects that arise due to competition for a limited substrate surface area. This study provides a model for understanding oligourethane surface coatings and demonstrates the importance of molecular structure and hydrogen bonding in determining adsorption behavior.

Two-dimensional crystallization: self-assembly, pseudopolymorphism, and symmetry-independent molecules.

The self-assembly of a series of 1,3-disubstituted benzenes has been scrutinized by scanning tunneling microscopy (STM) and computational modeling. Small changes in the functional groups (e.g., ester, thioester, ketone) resulted in dramatic changes in packing patterns. Remarkably, several of the molecules gave rise to monolayers with more than one molecule in the asymmetric unit and displayed multiple packing patterns. This constitutes the most complex behavior observed to date in this type of monolayer and illuminates several issues of importance in three-dimensional crystallization. Intermolecular interactions associated with the observation of multiple molecules in the asymmetric unit and stabilization of pseudopolymorphs were identified. The geometry and electrostatic properties of the isolated molecule and monolayer density were found to be critical in determining which packing motif was adopted.