This paper describes the fabrication of polymer stamps using DNA nanostructure templates. This process creates stamps having diverse nanoscale features with dimensions ranging from several tens of nanometers to micrometers. DNA nanostructures including DNA nanotubes, stretched λ-DNA, two-dimensional (2D) DNA brick crystals with three-dimensional (3D) features, hexagonal DNA 2D arrays, and triangular DNA origami were used as master templates to transfer patterns to poly(methyl methacrylate) and poly(l-lactic acid) with high fidelity. The resulting polymer stamps were used as molds to transfer the pattern to acryloxy perfluoropolyether polymer. This work establishes an approach to using self-assembled DNA templates for applications in soft lithography.Keywords: DNA nanostructures; nanofabrication; nanoimprint lithography; pattern transfer; polymer stamp; replica molding; self-assembly;

ConspectusGraphitic carbons are important solid materials with myriad applications including electrodes, adsorbents, catalyst support, and solid lubricants. Understanding the interaction between water and graphitic carbons is critically important for both fundamental material characterization and practical device fabrication because the water–graphitic interface is essential to many applications. Research interests in graphene and carbon nanotubes over the past decades have brought renewed interest to elucidate wettability of graphitic carbons and understand their interaction with the surrounding environment.Research on this topic can be traced back to the 1940s, and the prevailing notion has been that graphitic carbons are hydrophobic. Though there have been different voices, this conclusion is supported by many previous water contact angle tests and well accepted by the community since sp2 carbon is nonpolar in nature. However, recent results from our groups showed that graphitic surfaces are intrinsically mildly hydrophilic and adsorbed hydrocarbon contaminants from the ambient air render the surface hydrophobic. This unexpected finding challenges the long-lasting conception and could completely change the way graphitic materials are made, modeled, and modified. With several other research groups reporting similar findings, it is important for the community to realize the importance of airborne contamination on the surface-related properties of graphitic materials and revisit the intrinsic water–graphite interaction.This Account aims to summarize our recent work on water wettability of graphitic surfaces and discuss future research directions toward understanding the intrinsic water–graphite interaction. Historical perspective will first be provided highlighting the long accepted notion that graphite is hydrophobic along with a few reports suggesting otherwise. Next, our recent experimental data will be presented showing that pristine graphene and graphite are mildly hydrophilic; chemical analysis showed that hydrocarbons adsorb onto the clean surfaces thus rendering them hydrophobic. These results are further rationalized by analyzing the change in surface energy of the graphitic surfaces before and after hydrocarbon contamination. Facile methods to remove hydrocarbons from a contaminated surface will be discussed along with a convenient water treatment method that we developed to inhibit hydrocarbon adsorption onto a pristine graphitic surface. Implications of contamination will be illustrated through comparing the electrochemical activity of pristine and contaminated graphite. Lastly, consequences of these findings and future research directions to address a few important unanswered questions will be discussed.

Graphitic carbons are long regarded as model hydrophobic materials. However, recent work has shown that graphite and graphene are much more hydrophilic than previously thought. It was revealed that the commonly observed hydrophobic nature of graphite is due to airborne hydrocarbon contamination that was not considered in previous studies. This perspective highlights these recent developments and discusses their implications to research on water–carbon interactions, wetting transparency, electrochemistry, adsorption and adhesion, and lubrication and wear.Download high-res image (81KB)Download full-size image

This paper reviews the effect of ambient exposure on the properties of selected 2D materials. Many molecules in ambient air can adsorb onto 2D material surfaces to impact their properties and device performance. This paper highlights recent work on the interaction between 2D materials and three ambient-present molecules: O2, H2O, and airborne hydrocarbons. We focus our discussions on graphene but also include research on other 2D materials, such as BN, transition metal dichalcogenides, and 2D heterostructures. We discuss the molecular mechanism of their interactions with 2D materials and the impact on electrical, optical, and wetting properties and device performances.

DNA nanostructures are ideal templates for bottom-up assembly and fabrication of nanomaterials. Their structures can be tailored for a given application and modified with pinpoint precision. They offer the best of top down and bottom up assembly. We highlight recent progress in DNA nanotechnology and in particular advances that are relevant to the materials chemistry community. Examples of using DNA nanostructure to address materials chemistry challenges are highlighted.

We describe a method to pattern arbitrary-shaped silane self-assembled monolayers (SAMs) with nm scale resolution using DNA nanostructures as templates. The DNA nanostructures assembled on a silicon substrate act as a soft-mask to negatively pattern SAMs. Mixed SAMs can be prepared by back filling the negative tone patterns with a different silane.

In this report we show that the photochemical oxidation of monolayer graphene is strongly dependent on its underlying substrate. It was found that chemical vapor deposition-grown monolayer graphene transferred onto a silicon wafer is more easily oxidized compared to graphene that was left on the copper substrate or transferred onto a H2-annealed copper foil. The differences in the degree of oxidation were tentatively attributed to the varying energies of adhesion between the graphene and the underlying substrate. Our result has significant implications for the outdoor use of graphene as a transparent conducting material.

In this upper-level undergraduate experiment, students utilize micro-Raman spectroscopy to characterize graphene prepared by mechanical exfoliation and chemical vapor deposition (CVD). The mechanically exfoliated samples are prepared by the students while CVD graphene can be purchased or obtained through outside sources. Owing to the intense Raman signal of a few-layer graphene on a 300 nm thermal oxide silicon wafer, students can learn how different instrumental parameters used in Raman microscopy affect the quality of the measurement. This experiment gives students a first-hand experience in the production of a two-dimensional nanomaterial and exposes them to the utility of micro-Raman spectroscopy as a characterization technique.Keywords: Analytical Chemistry; Hands-on Learning/Manipulatives; Laboratory Instruction; Materials Science; Nanotechnology; Physical Chemistry; Raman Spectroscopy; Upper-Division Undergraduate;

The intrinsic wettability of graphitic materials, such as graphene and graphite, can be readily obscured by airborne hydrocarbon within 5–20 min of ambient air exposure. We report a convenient method to effectively preserve a freshly prepared graphitic surface simply through a water treatment technique. This approach significantly inhibits the hydrocarbon adsorption rate by a factor of ca. 20×, thus maintaining the intrinsic wetting behavior for many hours upon air exposure. Follow-up characterization shows that a nanometer-thick ice-like water forms on the graphitic surface, which remains stabilized at room temperature for at least 2–3 h and thus significantly decreases the adsorption of airborne hydrocarbon on the graphitic surface. This method has potential implications in minimizing hydrocarbon contamination during manufacturing, characterization, processing, and storage of graphene/graphite-based devices. As an example, we show that a water-treated graphite electrode maintains a high level of electrochemical activity in air for up to 1 day.Keywords: cleaning; contamination; cyclic voltammetry; graphene; graphite; spectroscopy; water adsorption;

DNA nanostructures are versatile templates for low cost, high resolution nanofabrication. However, due to the limited chemical stability of pure DNA structures, their applications in nanofabrication have long been limited to low temperature processes or solution phase reactions. Here, we demonstrate the use of DNA nanostructure as a template for high temperature, solid-state chemistries. We show that programmably shaped carbon nanostructures can be obtained by a shape-conserving carbonization of DNA nanostructures. The DNA nanostructures were first coated with a thin film of Al2O3 by atomic layer deposition (ALD), after which the DNA nanostructure was carbonized in low pressure H2 atmosphere at 800–1000 °C. Raman spectroscopy and atomic force microscopy (AFM) data showed that carbon nanostructures were produced and the shape of the DNA nanostructure was preserved. Conductive AFM measurement shows that the carbon nanostructures are electrically conductive.Keywords: DNA nanostructure; high temperature chemistry; shape-conserving carbonization

We report a mechanistic study of a DNA-mediated vapor phase HF etching of SiO2. The kinetics of SiO2 etching was studied as a function of the reaction temperature, time, and partial pressures of H2O, HF, and 2-propanol. Our results show that DNA locally increases the etching rate of SiO2 by promoting the adsorption of water and that the enhancement effect mostly originates from the organic components of DNA. On the basis of the mechanistic studies, we identified conditions for high-contrast (>10 nm deep), high-resolution (∼10 nm) pattern transfers to SiO2 from DNA nanostructures as well as individual double-stranded DNA. These SiO2 patterns were used as a hard mask for plasma etching of Si to produce even higher-contrast patterns that are comparable to those obtained by electron-beam lithography.

This paper reports a detailed mechanistic study of the effect of alkylamine on the synthesis of CdSe nanocrystals. Alkylamines are one of the most important additives for the synthesis of colloidal semiconductor nanocrystals. However, their effect on the monomer production as well as nanocrystal nucleation and growth are not well understood, as indicted by inconsistent and contradictory conclusions in the literature. We found that alkylamines slow down the reaction between cadmium oleate and trialkyl phosphine selenide by binding to cadmium and preventing the activation of trialkyl phosphine selenide. A linear correlation was observed between the observed reaction rate constant and the 31P NMR chemical shift or 1JP–Se of phosphine selenide. In the presence of alkylamine, an alkylaminophosphonium intermediate was observed. Mechanistic study suggests that the cleavage of P═Se bond is through nucleophilic attack by carboxylate instead of alkylamine. Interestingly, although alkylamines decrease the rate of monomer production, it increases the rate of CdSe nanocrystal growth. Although seemingly contradictory, this is due to a drastic decrease in the nanocrystal nucleation events in the presence of alkylamines. As a result, each nucleus is fed with more monomers and grows faster in the presence of alkylamine than in its absence.

This paper reports a high-throughput, label-free technique to visualize individual carbon nanotubes (CNTs) on a silicon wafer using a conventional optical microscope. We show that individual CNTs can locally enhance the rate of vapor-phase HF etching of SiO2 to produce a SiO2 trench that is several to several tens of nanometers in depth. The trench is visible under an optical microscope due to a change in the optical interference in the SiO2 layer, allowing the location of an individual CNT to be determined. With this technique, we demonstrate high-throughput Raman characterization and reactivity studies on individual CNTs.

We report the effect of chemical and physical treatments on the structural stability of DNA origami nanostructures. Our result shows that DNA nanostructure maintains its shape under harsh processing conditions, including thermal annealing up to 200 °C for 10 min, immersing in a wide range of organic solvents for up to 24 h, brief exposure to alkaline aqueous solutions, and 5 min exposure to UV/O3. Our result suggests that the application window of DNA nanostructure is significantly wider than previously believed.

There are significant controversies on the antibacterial properties of graphene oxide (GO): GO was reported to be bactericidal in saline, whereas its activity in nutrient broth was controversial. To unveil the mechanisms underlying these contradictions, we performed antibacterial assays under comparable conditions. In saline, bare GO sheets were intrinsically bactericidal, yielding a bacterial survival percentage of <1% at 200 μg/mL. Supplementing saline with ≤10% Luria–Bertani (LB) broth, however, progressively deactivated its bactericidal activity depending on LB-supplementation ratio. Supplementation of 10% LB made GO completely inactive; instead, ∼100-fold bacterial growth was observed. Atomic force microscopy images showed that certain LB components were adsorbed on GO basal planes. Using bovine serum albumin and tryptophan as well-defined model adsorbates, we found that noncovalent adsorption on GO basal planes may account for the deactivation of GO’s bactericidal activity. Moreover, this deactivation mechanism was shown to be extrapolatable to GO’s cytotoxicity against mammalian cells. Taken together, our observations suggest that bare GO intrinsically kills both bacteria and mammalian cells and noncovalent adsorption on its basal planes may be a global deactivation mechanism for GO’s cytotoxicity.Keywords: adsorption; antimicrobial; cytotoxicity; graphene; mechanism

We report an ultrasound exfoliation of graphite in a weakly basic solution to produce multi-layer graphene dispersion. A unique feature of this process is that no surfactant was added to stabilize the exfoliated graphene in water. The concentration of the graphene dispersion prepared by this approach can be up to 0.02 mg mL−1 and it was stable at room temperature for several months.

We report the effect of airborne hydrocarbon contamination on the water wettability of graphite. Graphite is traditionally known to be hydrophobic with water contact angle (WCA) within the 75–95° range. We found that the WCA of highly ordered pyrolytic graphite (HOPG) was 64.4 ± 2.9° when measured within 10 s after exfoliation in air and increased to ca. 90° after exposure to the ambient air. Ellipsometry measurement showed growth of an adsorptive layer on exfoliated HOPG and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) data indicated that the layer is airborne hydrocarbon. Theoretical calculation confirms that adsorption of only a monolayer amount of hydrocarbon is enough to reproduce the hydrophobic behavior previously observed on HOPG. These results indicated that graphite is intrinsically more hydrophilic than previously believed and that surface adsorbed airborne hydrocarbon is the source of hydrophobicity.

We report an unusual enrichment and assembly of TiO2 nanocrystals at water/hydrophobic interfaces through oxidative hydrolysis of TiCl3 in water. The assembly is a spontaneous process that involves on-water inorganic reaction and assembly in the absence of any organic phases. In this process, TiO2 nanoparticles are preferentially produced at water/hydrophobic interfaces. When the surface tension of the aqueous phase is above a critical value, ca. 25–35 mN m–1, these TiO2 nanocrystals can spontaneously accumulate at water/air interfaces to produce macroscopic sized sheets and tubes.

Because of the atomic thinness of graphene, its integration into a device will always involve its interaction with at least one supporting substrate, making the surface energy of graphene critical to its real-life applications. In the current paper, the contact angle of graphene synthesized by chemical vapor deposition (CVD) was monitored temporally after synthesis using water, diiodomethane, ethylene glycol, and glycerol. The surface energy was then calculated based on the contact angle data by the Fowkes, Owens–Wendt (extended Fowkes), and Neumann models. The surface energy of fresh CVD graphene grown on a copper substrate (G/Cu) immediately after synthesis was determined to be 62.2 ± 3.1 mJ/m2 (Fowkes), 53.0 ± 4.3 mJ/m2 (Owens–Wendt) and 63.8 ± 2.0 mJ/m2 (Neumann), which decreased to 45.6 ± 3.9, 37.5 ± 2.3, and 57.4 ± 2.1 mJ/m2, respectively, after 24 h of air exposure. The ellipsometry characterization indicates that the surface energy of G/Cu is affected by airborne hydrocarbon contamination. G/Cu exhibits the highest surface energy immediately after synthesis, and the surface energy decreases after airborne contamination occurs. The root cause of intrinsically mild polarity of G/Cu surface is discussed.

Recent advances in DNA nanotechnology have made it possible to construct DNA nanostructures of almost arbitrary shapes with 2–3 nm of precision in their dimensions. These DNA nanostructures are ideal templates for bottom-up nanofabrication. This review highlights the challenges and recent advances in three areas that are directly related to DNA-based nanofabrication: (1) fabrication of large scale DNA nanostructures; (2) pattern transfer from DNA nanostructure to an inorganic substrate; and (3) directed assembly of DNA nanostructures.

We describe a method to form custom-shaped inorganic oxide nanostructures by using DNA nanostructure templates. We show that a DNA nanostructure can modulate the rate of chemical vapor deposition of SiO2 and TiO2 with nanometer-scale spatial resolution. The resulting oxide nanostructure inherits its shape from the DNA template. This method generates both positive-tone and negative-tone patterns on a wide range of substrates and is compatible with conventional silicon nanofabrication processes. Our result opens the door to the use of DNA nanostructures as general-purpose templates for high-resolution nanofabrication.

We survey the chemical reactions between common precursors used in the synthesis of metal chalcogenide nanocrystals and outline how they affect the mechanism and kinetics of nanocrystal growth. We emphasize syntheses of cadmium selenide and cadmium sulfide where a variety of metal and chalcogenide precursors have been explored, though this is supplemented by studies of zinc and lead chalcogenide formation where appropriate. This review is organized into three sections, highlighting kinetics, metal precursors, and chalcogenide precursors, respectively. Section I is dedicated to the role of precursor conversion as a source of monomers and the importance of the supply rate on nanocrystal nucleation and growth. Section II describes the structure and reactivity of cadmium carboxylates, phosphonates, and chalcogenolates. Section III describes the reaction chemistry of commonly employed chalcogenide precursors and the mechanisms by which they react with metal precursors.Keywords: cadmium chalcogenide; II−VI; mechanism; precursor conversion; quantum dot;

Diffusion-ordered spectroscopy (DOSY) was used to investigate the solution structure of cadmium carboxylate. The molecular weights of cadmium complexes highly depend on the solvent; the complexes are polymeric in toluene but break up in the presence of polar solvents or coordinating ligands.

•The effect of a nano-lubricant on the friction and wear of CVD graphene was studied.•Lubricating Graphene/Si results in lower friction but higher wear.•Lubricating Ggraphene/Ni results in lower wear but unchanged friction.•The mechanisms were discussed based on the roughness and interfacial adhesion.Due to its atomic thickness (thinness), the wear of graphene in nanoscale devices or as a protective coating is a serious concern. It is highly desirable to develop effective methods to reduce the wear of graphene. In the current paper, the effect of a nano-lubricant, perfluoropolyether, on the wear of graphene on different substrates is investigated. Graphene was synthesized by chemical vapor deposition (CVD) and characterized by Raman spectroscopy. The nano-lubricant is applied on the graphene by dip-coating. The friction and wear of graphene samples are characterized by nanotribometer, AFM, optical microscopy and Raman spectroscopy. The results showed that lubricating silicon/graphene with nano-lubricant reduces the friction but increases the wear. However, lubricating nickel/graphene with nano-lubricant has little effect on the friction but reduce the wear significantly. The underlying mechanism has been discussed on the basis of the graphene–substrate adhesion and the roughness. The current study provides guidance to the future design of graphene-containing devices.

This paper reports the enhancement of long-term oxidation of copper at room temperature by a graphene coating. Previous studies showed that graphene is an effective anticorrosion barrier against short-term thermal and electrochemical oxidation of metals. Here, we show that a graphene coating can, on the contrary, accelerate long-term oxidation of an underlying copper substrate in ambient atmosphere at room temperature. After 6 months of exposure in air, both Raman spectroscopy and energy-dispersive X-ray spectroscopy indicated that graphene-coated copper foil had a higher degree of oxidation than uncoated foil, although X-ray photoelectron spectroscopy showed that the surface concentration of Cu2+ was higher for the uncoated sample. In addition, we observed that the oxidation of graphene-coated copper foil was not homogeneous and occurred within micrometer-sized domains. The corrosion enhancement effect of graphene was attributed to its ability to promote electrochemical corrosion of copper.Keywords: copper; corrosion; graphene; long-term; oxidation

We outline a reaction pathway for the cleavage of the P═Se bond in trialkylphosphine selenide during the synthesis of CdSe nanocrystals. The reaction between cadmium carboxylate and trimethylphosphine selenide in the presence of an alcohol produces alkoxytrimethylphosphonium (2). Control experiments and density functional theory calculations suggested that the cleavage of the P═Se bond is initiated by nucleophilic attack of carboxylate on a Cd2+-activated phosphine selenide to produce an acyloxytrialkylphosphonium intermediate (1), which is converted to 2 in the presence of an alcohol.

We report the atmospheric thermal oxidation of chemical vapor deposition (CVD)-grown graphene and its effect on the wrinkles in graphene. Heating CVD-grown single-layer graphene in air at 550 °C leads to the disappearance of the wrinkles and the formation of nanoscale cracks and pits in the basal plane. Under this reaction condition, the wrinkles were not preferentially attacked by O2, which we attribute to the relaxation of wrinkles as well as the presence of more reactive point defects and grain boundaries in the basal plane. Randomly stacked two-layer graphene was found to be more stable toward oxidation than was single-layer graphene.

We demonstrate a new approach to pattern transfer for bottom-up nanofabrication. We show that DNA promotes/inhibits the etching of SiO2 at the single-molecule level, resulting in negative/positive tone pattern transfers from DNA to the SiO2 substrate.

We report an ultrasound exfoliation of graphite in a weakly basic solution to produce multi-layer graphene dispersion. A unique feature of this process is that no surfactant was added to stabilize the exfoliated graphene in water. The concentration of the graphene dispersion prepared by this approach can be up to 0.02 mg mL−1 and it was stable at room temperature for several months.

Recent advances in DNA nanotechnology have made it possible to construct DNA nanostructures of almost arbitrary shapes with 2–3 nm of precision in their dimensions. These DNA nanostructures are ideal templates for bottom-up nanofabrication. This review highlights the challenges and recent advances in three areas that are directly related to DNA-based nanofabrication: (1) fabrication of large scale DNA nanostructures; (2) pattern transfer from DNA nanostructure to an inorganic substrate; and (3) directed assembly of DNA nanostructures.

Diffusion-ordered spectroscopy (DOSY) was used to investigate the solution structure of cadmium carboxylate. The molecular weights of cadmium complexes highly depend on the solvent; the complexes are polymeric in toluene but break up in the presence of polar solvents or coordinating ligands.

We describe a method to pattern arbitrary-shaped silane self-assembled monolayers (SAMs) with nm scale resolution using DNA nanostructures as templates. The DNA nanostructures assembled on a silicon substrate act as a soft-mask to negatively pattern SAMs. Mixed SAMs can be prepared by back filling the negative tone patterns with a different silane.