Luminescence quantum efficiency of nanocrystalline ZnS:Mn\u3csup\u3e2+\u3c/sup\u3e: 1. Surface passivation and Mn\u3csup\u3e2+\u3c/sup\u3e concentration

\u3cp\u3eThe luminescence quantum efficiency of nanocrystalline ZnS:Mn\u3csup\u3e2+\u3c/sup\u3e is studied to provide a better understanding on how the quantum efficiency is influenced by the Mn\u3csup\u3e2+\u3c/sup\u3e concentration, the nature of the passivating polymer, and the synthesis conditions. The results show an increase of the luminescence quantum efficiency with the Mn\u3csup\u3e2+\u3c/sup\u3e concentration in the nanocrystals for very low Mn\u3csup\u3e2+\u3c/sup\u3e concentrations. Between 0.3 and 1.5 at. % Mn\u3csup\u3e2+\u3c/sup\u3e the increase in quantum efficiency levels off, to reach an almost constant level between 1.5 and 5.6 at. % Mn\u3csup\u3e2+\u3c/sup\u3e. Up to a concentration of 5.6 at. %, no concentration quenching is observed. The influence of the nature of the passivating polymer is investigated by comparing the luminescence quantum efficiencies for nanoparticles coated with poly(vinylbutyral) (PVB), poly(vinyl alcohol) (PVA), methacrylic acid (MA), or sodium polyphosphate (PP) or without a passivating polymer. For the presently used synthesis method (in water), the highest quantum efficiencies (around 4%) are obtained for nanocrystalline ZnS:Mn\u3csup\u3e2+\u3c/sup\u3e capped with PP. Nanoparticles synthesized in a nitrogen atmosphere have higher quantum yields than nanoparticles made in ambient air. In general, large variations in luminescence properties are observed due to unintentional variations in the synthesis conditions. For research on the luminescence properties and quantum efficiencies of nanocrystalline ZnS:Mn\u3csup\u3e2+\u3c/sup\u3e, it is very important to check the reproducibility of results, to standardize synthesis conditions, and to measure absolute quantum efficiencies rather than relative changes in luminescence intensity.\u3c/p\u3

Luminescence quantum efficiency of nanocrystalline ZnS:Mn\u3csup\u3e2+\u3c/sup\u3e: 2. Enhancement by UV irradiation

\u3cp\u3eIn this paper the influence of UV irradiation on the luminescence quantum efficiency of nanocrystalline ZnS:Mn\u3csup\u3e2+\u3c/sup\u3e is studied. Samples passivated with poly(vinylbutyral) (PVB), poly(vinyl alcohol) (PVA), methacrylic acid (MA), and sodium polyphosphate (PP) and samples without passivating polymer were UV irradiated in air, nitrogen, dry air, and wet nitrogen. Samples coated with PVB, MA, and PVA show the highest increase of the luminescence quantum efficiency upon irradiation, due to UV curing of the passivating polymer. The UV enhancement observed for the PP-coated sample and the unpassivated sample is explained by photochemical reactions that take place at the surface of the nanoparticles during the irradiation. The products of these reactions (e.g., ZnSO\u3csub\u3e4\u3c/sub\u3e or Zn(OH)\u3csub\u3e2\u3c/sub\u3e) serve as a passivating layer around the nanoparticles, which increases the quantum efficiency. The highest steady-state quantum efficiencies obtained after UV curing are around 10%. In addition to the long-term UV enhancement of the Mn\u3csup\u3e2+\u3c/sup\u3e emission intensity, short-term (seconds) UV-induced quenching of the Mn\u3csup\u3e2+\u3c/sup\u3e emission is reported and a model for the quenching is discussed.\u3c/p\u3

Long-lived emission in nanocrystalline

\u3cp\u3eRecently, it has been reported that doped semiconductor nanoparticles can yield both high luminescence efficiencies and a spectacular lifetime shortening, which suggests that doped semiconductor nanoparticles form a new class of luminescent materials for various applications. From lifetime measurements and time-resolved spectroscopy we conclude that the (Formula presented) emission does not show a spectacular shortening of the decay time upon decreasing particle size as reported earlier. The luminescence of nanocrystalline (Formula presented) indeed has a short decay time (∼100 ns), but also shows a long ms range decay time. The short decay time is ascribed to a defect-related emission of ZnS, and is not from the decay of the (Formula presented) transition of the (Formula presented) impurity as suggested by other authors. The (Formula presented) transition of the (Formula presented) has a “normal” decay of about 1.9 ms. Based on our observations, we conclude that doped semiconductor nanoparticles do not form a new class of luminescent materials, combining a high efficiency with a short (ns) decay time.\u3c/p\u3

X-ray photoelectron spectroscopy study on Fe and Co catalysts during the first stages of ethanol chemical vapor deposition for single-walled carbon nanotube growth

Optimized chemical vapor deposition processes for single-walled carbon nanotube (SWCNT) can lead to the growth of dense, vertically aligned, mm-long forests of SWCNTs. Precise control of the growth process is however still difficult, mainly because of poor understanding of the interplay between catalyst, substrate and reaction gas. In this paper we use x-ray photoelectron spectroscopy (XPS) to study the interplay between Fe or Co catalysts, SiO2 and Al2O3 substrates and ethanol during the first stages of SWCNT forest growth. With XPS we observe that ethanol oxidizes Fe catalysts at carbon nanotube (CNT) growth temperatures, which leads to reduced carbon nanotube growth. Ethanol needs to be decomposed by a hot filament or other technique to create a reducing atmosphere and reactive carbon species in order to grow vertically aligned single-walled carbon nanotubes from Fe catalysts. Furthermore, we show that Al2O3, unlike SiO2, plays an active role in CNT growth using ethanol CVD. From our study we conclude that metallic Fe on Al2O3 is the most optimal catalyst/substrate combination for high-yield SWCNT forest growth, using hot filament CVD with ethanol as the carbon containing gas

Strategies to facilitate the formation of free standing MoS2 nanolayers on SiO2 surface by atomic layer deposition: a DFT study

In this study, we employ density functional theory calculations to investigate the very initial formation of a buffer layer during atomic layer deposition of MoS2 at the SiO2 (001) surface. In our previous study, we described that the self-limiting atomic layer deposition (ALD) reactions using Mo(NMe2)2(NtBu)2 as precursor and H2S as co-reagent terminate in the formation of a so-called building block on the SiO2 (001) surface. This building block consists of Mo which shares bonds with the surface O of SiO2 (001) at the bottom and terminal S at the top. Electronic band structure calculations indicate that the subsequently deposited buffer-layer that is composed of these building blocks has (opto)-electrical properties that are far from the ideal situation. Based on our studies, we propose alternative ALD chemistries which lead to the formation of a so-called underpinned building block. In this cluster, the Mo atoms are underpinned by S atoms, suppressing the formation of a buffer layer. This ultimately facilitates the formation of a free standing conformal 2D-MoS2 nanolayer at the interface. Through the proposed chemistries, the opto-electrical properties of the\u3cbr/\u3edeposited layers will be preserved

Diamond-like-carbon LC-alignment layers for application in LCOS microdisplays

\u3cp\u3eTo improve the lifetime and yield of LCOS microdisplays, non-contact LC alignment techniques using inorganic materials are under investigation. This report focuses on oblique ion-beam treatment of diamond-like carbon (DLC) layers, and in particular on the influence of the ion dose on the LC alignment on DLC, keeping the ion-beam angle (40°) and ion-beam energy (170 eV) the same. LC alignment on ion-milled DLC layers is uniform if the ion dose is between 3.8 × 10\u3csup\u3e-4\u3c/sup\u3e C/cm\u3csup\u3e2\u3c/sup\u3e and 5.5×10 \u3csup\u3e-3\u3c/sup\u3e C/cm\u3csup\u3e2\u3c/sup\u3e. Above and below this ion dose range, non-uniform alignment is observed. NEXAFS experiments show that this is caused by lack of molecular anisotropy on the surface of the ion-milled DLC layers. By varying the ion dose between 3.8 × 10\u3csup\u3e-4\u3c/sup\u3e C/cm\u3csup\u3e2\u3c/sup\u3e and 5.5 × 10\u3csup\u3e-3\u3c/sup\u3e C/cm\u3csup\u3e2\u3c/sup\u3e, LC molecules have an average pre-tilt between 3° and 5°, which is within the desired range for application in LCOS microdisplays. The lifetime of the LCOS microdisplays with ion-milled DLC for projection-TV application is, however, shorter than the lifetime of microdisplays with Pl layers. Ion milling probably creates a reactive surface that is unstable under the high light fluxes used in projection TVs. A solution for this problem could be chemical passivation of the ion-milled alignment layers. Initial experiments with passivation of ion-milled Pl resulted in an increase in lifetime, but the lifetime after passivation was still lower than the lifetime of rubbed Pl layers (factor 0.7). Nevertheless, ion-milling of DLC or Pl can be a good alternative LC alignment technique in other LCD applications. LC-alignment layers based on inorganic layers such as obliquely deposited SiO\u3csub\u3e2\u3c/sub\u3e films would be a better option for application in LCOS microdisplays due to their higher light stability.\u3c/p\u3

On the incorporation of trivalent rare earth ions in II-VI semiconductor nanocrystals

\u3cp\u3eNanocrystalline ZnS and CdS samples have been synthesized in the presence of Eu\u3csup\u3e3+\u3c/sup\u3e and Tb\u3csup\u3e3+\u3c/sup\u3e using various techniques (precipitation in water, methanol, or toluene and inverse micelle techniques) which have been reported to yield ZnS or CdS nanoparticles doped with luminescent rare earth ions. Nanocrystalline particles with a typical diameter of 4 nm were formed. In some cases, particles were heated (up to 800 °C) which resulted in an increase of the particle diameter (> 20 nm). To study the incorporation of rare earth ions in the particles, luminescence spectra have been measured. Upon excitation in the semiconductor host lattice, no emission or weak emission is observed from the rare earth ions. The excitation spectra of the characteristic rare earth emissions show excitation lines corresponding to intraconfigurational 4f\u3csup\u3en\u3c/sup\u3e-4f\u3csup\u3en\u3c/sup\u3e transitions of the rare earth ions but not the semiconductor host lattice excitation band. The absence of a host lattice excitation band indicates that with the presently used synthesis techniques the rare earth ions are not incorporated in the nanocrystalline semiconductor particles but are probably adsorbed at the surface.\u3c/p\u3

Initial stage of atomic layer deposition of 2D-MoS\u3csub\u3e2\u3c/sub\u3e on a SiO\u3csub\u3e2\u3c/sub\u3e surface:A DFT study

\u3cp\u3eIn this study, we investigate the reactions involving Atomic Layer Deposition (ALD) of 2D-MoS\u3csub\u3e2\u3c/sub\u3e from the heteroleptic precursor Mo(NMe\u3csub\u3e2\u3c/sub\u3e)\u3csub\u3e2\u3c/sub\u3e(N\u3csup\u3et\u3c/sup\u3eBu)\u3csub\u3e2\u3c/sub\u3e and H\u3csub\u3e2\u3c/sub\u3eS as the co-reagent on a SiO\u3csub\u3e2\u3c/sub\u3e(0001) surface by means of density functional theory (DFT). All dominant reaction pathways from the early stage of adsorption of each ALD reagent to the formation of bulk-like Mo and S at the surface are identified. In the metal pulse, proton transfer from terminal OH groups on the SiO\u3csub\u3e2\u3c/sub\u3e to the physisorbed metal precursor increases the Lewis acidity of Mo and Lewis basicity of O, which gives rise to the chemical adsorption of the metal precursor. Proton transfer from the surface to the dimethylamido ligands leads to the formation and desorption of dimethylamine. In contrast, the formation and desorption of tert-butylamine is not energetically favorable. The tert-butylimido ligand can only be partially protonated in the metal pulse. In the sulphur pulse, co-adsorption and dissociation of H\u3csub\u3e2\u3c/sub\u3eS molecules give rise to the formation and desorption of tert-butylamine. Through the calculated activation energies, the cooperation between H\u3csub\u3e2\u3c/sub\u3eS molecules ('cooperative' mechanism) is shown to have a profound influence on the formation and desorption of tert-butylamine, which are crucial steps in the initial ALD deposition of 2D-MoS\u3csub\u3e2\u3c/sub\u3e on SiO\u3csub\u3e2\u3c/sub\u3e. The cyclic ALD reactions give rise to the formation of a buffer layer which might have important consequences for the electrical and optical properties on the 2D layer formed in the subsequent homodeposition.\u3c/p\u3

Large low-frequency resistance noise in chemical vapor deposited graphene

We report a detailed investigation of resistance noise in single layer graphene films on Si/SiO2 substrates obtained by chemical vapor deposition (CVD) on copper foils. We find that noise in these systems to be rather large, and when expressed in the form of phenomenological Hooge equation, it corresponds to Hooge parameter as large as 0.1–0.5. We also find the variation in the noise magnitude with the gate voltage (or carrier density) and temperature to be surprisingly weak, which is also unlike the behavior of noise in other forms of graphene, in particular those from exfoliation