Modified Organosilane Monolayers With Enhanced Radiation Stability.

Organosilane monolayers containing long carbon chains are susceptible to damage when exposed to X-rays and other radiation during characterization or processing. The origin of the damage has been attributed to both energetic photons and photoelectrons. These particles can break bonds in the molecules comprising the layer altering its composition and structure and can create silanol groups at the interface between the monolayer and the silicon dioxide surface. A common practice to minimize damage is to keep the X-ray beam time as short as possible. In this study, we report a method to deposit an organosilane monolayer on silicon dioxide that withstands X-ray beam damage to the interface. Adding the small monofunctional trimethylchlorosilane to a layer composed of the long-chain trifunctional octadecyltrichlorosilane prevented X-ray beam damage for at least 4 h. Interfacial damage was monitored by titrating silanol groups with TiCl4 and water atomic layer deposition and measuring the TiO2 deposited by X-ray photoelectron spectroscopy (XPS). The small monofunctional silane reacted with residual silanol groups capping the siloxane (-Si-O-Si-O-) chains at the interface. The low silanol group concentration minimized X-ray beam damage and prevented TiO2 from depositing to the detection limit of XPS. Using a small molecule to terminate reactive groups could improve the stability of monolayers not only during characterization and processing, but also when the monolayer is a component in an electrical device by reducing sources of interfacial charge traps.

Detecting and Removing Defects in Organosilane Self-Assembled Monolayers.

Defects occur as self-assembled monolayers form, and the number and type of defects depend on the surface preparation and deposition solvent, among other parameters. Indirect measures to detect defects using a layer property, such as the thickness or bond vibrational frequency, are used routinely for process development but often lack sensitivity. Direct measures using an atomic probe offer a glimpse of defect structures but over a small fraction of the layer. Direct detection after reacting defects by etching or deposition is more common, and this approach has advanced our understanding of how monolayers form and has led to improved monolayers for a variety of applications. Here we show that a series of TiCl4 gas pulses reacts with defects in organosilane layers on SiO2 depositing TiO, which was measured by X-ray photoelectron spectroscopy. The defects were silanol groups and siloxane bridge bonds at the interface between the layer and the SiO2 surface and on agglomerates physisorbed to the layer. As the TiO saturation coverage or the total number of defects decreased, the incubation period in which no TiO was detected became longer. Cleaning the layer by solvent extraction to remove nonpolar agglomerates followed by an aqueous mixture of ammonium hydroxide and hydrogen peroxide, which is Standard Clean 1, a common particle removal step for silicon surfaces, produced an organosilane monolayer without agglomerates based on atomic force microscopy. After a second organosilane immersion, the monolayer density rose to 3.8 molecules/nm2. This monolayer inhibited the deposition of TiO on the SiO2 surface for 250 pulses of TiCl4 and 200 complete TiO2 atomic layer deposition cycles using TiCl4 and water vapor, and it failed at 300 complete cycles. The Standard Clean 1 solution not only removed defects left by solvent extraction but also led to the reorganization of the organosilane layer.

Ammonia photodissociation promoted by Si(100).

Using in situ X-ray photoelectron spectroscopy measurements after reaction, we show that hydrogen-terminated Si(100) perturbs the bonding of physisorbed NH3 enabling a photochemical decomposition pathway at wavelengths different from those characteristic of either the molecule in the gas phase or the semiconductor bandgap. UV illumination only of gas phase NH3 at partial pressures from 0.1 to 100 Torr produced a maximum at 10 Torr in the N surface coverage. This is in good agreement with a model of the radical production rate showing that at this pressure the gas density balances the flux of photons at the surface with energies sufficient to dissociate NH3. UV illumination of both the gas phase and the surface produced a monotonic increase in the N coverage with pressure as well as coverages that were 3-10 times higher than when only the gas phase was illuminated. The amine saturation coverage scaled with the UV fluence at 10 Torr and 75 °C, reaching 6.9 × 10(14) atoms/cm(2) (∼1 N atom per Si surface atom) at 19 mW/cm(2) and 12 × 10(14) atoms/cm(2) (∼1.8 N per Si) at 35 mW/cm(2). Monochromatic illumination showed that the wavelengths driving deposition were not correlated with the Si bandgap, but instead were roughly the same as gas phase photodissociation (λ < 220 nm). The primary driving force to replace the hydrogen termination with amine groups was direct photodissociation of NH3 molecules whose electronic structure was perturbed by interaction with the surface. Amine groups enhanced the surface reaction of water present as a contaminant in the source gas. These results show that molecules in weakly bound surface states can have a dramatic impact on the photochemistry.

Ligand-controlled growth of ZnSe quantum dots in water during Ostwald ripening.

A strong ligand effect was observed for the aqueous-phase growth of ZnSe quantum dots (QDs) in the Ostwald ripening (OR) stage. The QDs were made by injecting Se monomer at room temperature followed by a ramp to 100 °C. The ramp produced a second, more gradual increase in the concentrations of both Zn and Se monomers fed by the dissolution of QDs below the critical size. The dissolution process was followed using measurements of the mass of Zn in QDs and in the supernatant by inductively coupled plasma optical emission spectroscopy (ICP-OES). Despite the flux of monomers, there was little growth in the QDs of average size based on UV-vis absorption spectra, until the temperature reached 100 °C, when there was a period of rapid growth followed by a period of linear growth. The linear growth stage is the result of OR as the total mass of Zn in QDs and in the solvent remained constant. The growth data were fit to a continuum model for the limiting case of surface reaction control. The rate is proportional to the equilibrium coefficient for ligand detachment from the QD surface. The ligand 3-mercaptopropionic acid (MPA) was the most tightly bound to the surface and produced the lowest growth rate of (1.5-2) × 10(-3) nm/min in the OR stage, whereas thiolactic acid (TLA) was the most labile and produced the highest growth rate of 3 × 10(-3) nm/min. Methyl thioglycolate (MTG) and thioglycolic acid (TGA) produced rates in between these values. Ligands containing electron-withdrawing groups closer to the S atom and branching promote growth, whereas longer, possibly bidendate, ligands retard it. Mixed ligand experiments confirmed that growth is determined by ligand bonding strength to the QD. Photoluminescence spectroscopy showed that the more labile the ligand, the more facile the repair of surface defects during the exposure of the QDs to room light.

Simple colloidal synthesis of single-crystal Sb-Se-S nanotubes with composition dependent band-gap energy in the near-infrared.

We report the first synthesis of high-quality binary and ternary Sb(2)Se(3-x)S(x) nanotubes across the entire compositional range from x = 0 to 3 via a simple, low-cost, colloidal synthetic method of injection of Sb(III)-complex solution into a hot paraffin liquid containing Se, S, or a mixture thereof. In contrast to the classic rolling mechanism, the modular formation of the reported nanotubes follows a four-stage self-seeding process: (i) amorphous nanospheres, (ii) short crystalline nanotubes growing out of relatively large amorphous nanospheres, (iii) long crystalline nanotubes attached to small amorphous nanospheres, and (iv) single-crystal nanotubes. The obtained single-crystal nanotubes have tunable composition, orthorhombic phase, well-defined rectangular cross sections, and growth direction along [001], as revealed by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and selected area electron diffraction studies. UV-vis-NIR absorption spectroscopy reveals that the optical bandgap energy of the Sb(2)Se(3-x)S(x) (0 < or = x < or = 3) nanotubes increases quadratically with the sulfur concentration x with these bandgap energies falling in the range from 1.18 to 1.63 eV at the red edge of the solar spectrum. The present study opens a new avenue to low-cost, large-scale synthesis of high quality semiconductor nanotubes with technological applications in solar energy conversion and also for a wide range of optical nanodevices operating in the near-infrared.

Water-based route to ligand-selective synthesis of ZnSe and Cd-doped ZnSe quantum dots with tunable ultraviolet A to blue photoluminescence.

A water-based route has been demonstrated for synthesizing ZnSe and Cd-doped ZnSe (Zn(x)Cd(1-x)Se, 0 < x < 1) quantum dots (QDs) that have tunable and narrow photoluminescence (PL) peaks from the ultraviolet A (UVA) to the blue range (350-490 nm) with full-width at half-maximum (fwhm) values of 24-36 nm. Hydrazine (N(2)H(4)) was used to maintain oxygen-free conditions, allowing the reaction vessel to be open to air. The properties of the QDs were controlled using the thiol ligands, 3-mercaptopropionic acid (MPA), thiolglycolic acid (TGA), and l-glutathione (GSH). On the basis of optical spectra, linear three-carbon MPA attenuated nucleation and growth, yielding small ZnSe QDs with a high density of surface defects. In contrast, TGA and GSH produced larger ZnSe QDs with lower surface defect densities. The absorption spectra show that growth was more uniform and better controlled with linear two-carbon TGA than branched bifunctional GSH. After 20 min of growth TGA-capped ZnSe had an average diameter of 2.5 nm based on high-resolution transmission electron microscopy images; these nanocrystals had an absorbance peak maximum of approximately 340 nm (3.65 eV) and a band gap PL emission peak at 372 nm (3.34 eV). Highly fluorescent Zn(x)Cd(1-x)Se QDs were fabricated by adding a Cd-thiol complex directly to ZnSe QD solutions; PL peaks were tuned in the blue range (400-490 nm) by changing the Zn to Cd ratio. The Cd-bearing nanocrystals contained proportionally more Se based on X-ray photoelectron spectroscopy, and Cd-Se bonds had ionic character, in contrast to primarily covalent Zn-Se bonds.

Kinetics and mechanism for the reaction of hexafluoroacetylacetone with CuO in supercritical carbon dioxide.

A kinetic model and mechanism were developed for the heterogeneous chelation reaction of thin CuO films with hexafluoroacetylacetone (hfacH) in supercritical CO2. This reaction has relevance for processing nanoscale structures and, more importantly, serves as a model system to tune the reaction behavior of solids using supercritical fluids. Precise control over reaction conditions enabled accurate etching rates to be measured as a function of both temperature [(53.5-88.4) +/- 0.5 degrees C] and hfacH concentration (0.3-10.9 mM), yielding an apparent activation energy of 70.2 +/- 4.1 kJ/mol and an order of approximately 0.6 with respect to hfacH. X-ray photoelectron spectroscopy and scanning electron microscopy were used to characterize the CuO surface, and a maximum etching rate of 24.5 +/- 3.1 A/min was obtained. Solvation forces between hfacH and the dense CO2 permitted material removal at temperatures more than 100 degrees C lower than that of the analogous gas-phase process. In the low concentration regime, the etching reaction was modeled with a three-step Langmuir-Hinshelwood mechanism. Small amounts of excess water nearly doubled the reaction rate through the proposed formation of a hydrogen-bonded hfacH complex in solution. Further increases in the hfacH concentration up to 27.5 mM caused a shift to first-order kinetics and an adsorption-limited or Rideal-Eley mechanism. These results demonstrate that relatively modest increases in concentration can prompt a heterogeneous reaction in supercritical CO2 to switch from a mechanism most commonly associated with a low-flux gas to one emblematic of a high-flux liquid.

A new route to self-assembled tin dioxide nanospheres: fabrication and characterization.

Nearly monodispersed self-assembled tin dioxide (SnO2) nanospheres with intense photoluminescence (PL) were synthesized using a new wet chemistry technique. Instead of coprecipitating stannous salts, bulk tin (Sn) metal was oxidized at room temperature in a solution of hydrogen peroxide and deionized water containing polyvinylpyrrolidone (PVP) and ethylenediamine (EDA). SnO2 nanocrystals were produced with diameters of approximately 3.8 nm that spontaneously self-assembled into uniform SnO2 nanospheres with diameters of approximately 30 nm. Analysis was performed by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, high-resolution transmission electron microscopy, selected area electron diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, UV-vis absorption spectroscopy, PL spectroscopy, and fluorescence lifetime measurements. The SnO2 nanospheres displayed room-temperature purple luminescence with an intense band at 394 nm (approximately 3.15 eV) and a high quantum yield of approximately 15%, likely as a result of emission from the surface states of SnO2/PVP complexes. The present study could open a new avenue to large-scale synthesis of self-assembled functional oxide nanostructures with technological applications as purple emitters, biological labels, gas sensors, lithium batteries, and dye-sensitized solar cells.

Strong blue photoluminescence from single-crystalline bismuth oxychloride nanoplates.

Single-crystalline bismuth oxychloride (BiOCl) nanoplates with in-plane sizes of 200-500 nm and a thickness of 15-25 nm are synthesized by a simple solution route. Strong blue photoluminescence centred at 455 nm (∼2.72 eV) with very high quantum yields (Φ(PL)∼0.4) has been observed at room temperature, representing the first report of strong room temperature photoluminescence from bismuth oxyhalide nanomaterials. It is envisaged that bismuth oxychloride could join the family of non-cadmium based high-efficiency emitters; it has promising applications in various fields, especially in light emitting diodes, lasers and solar cells.

Spherical hexagonal tellurium nanocrystals: fabrication and size-dependent structural phase transition at high pressure.

Single-crystalline spherical nearly monodisperse tellurium (Te) nanocrystals (NCs) with average diameters of 20 and 90 nm, respectively, have been fabricated for the first time by a facile solution sonochemistry process. The structural characterizations show that the as-synthesized Te NCs have pure hexagonal structure, as revealed by x-ray diffraction (XRD), selected-area electron diffraction (SAED), energy-dispersive x-ray (EDX) spectroscopy, and high-resolution transmission electron microscopy (HRTEM) methods. The size-dependent structural phase transition of Te NCs up to the high pressure of 20 GPa has been investigated in a diamond anvil cell using resistance measurement at room temperature, and compared with the behavior of bulk Te under identical conditions. The experimental results indicate that 20 nm Te NCs, 90 nm NCs, and bulk Te all undergo two phase transitions up to 20 GPa, their respective transition pressures being about 7.2 and 10.3 GPa, 5.9 and 8.8 GPa, and 4.0 and 6.8 GPa. This indicates that the phase transition pressures are higher for the smaller NCs. In this paper we discuss the size-dependent structural phase transitions, the sluggishness of the phase transition process, and the fluctuating properties of the phase transition products at high pressure. The present work might open an avenue to real-time detection of the dynamics of the phase transition in bulk and nanoscale materials at high pressure, and also could serve as a guide to tailoring the microscopic properties of materials.