Chemical vapour deposited ZnO nanowires for detecting Ethanol and NO2

Randomly oriented ZnO nanowires were grown directly onto alumina substrates having platinum interdigitated screen-printed electrodes via the chemical vapor deposition method using Au as catalyst. Three different Au film thicknesses (i.e., 3, 6 or 12 nm) were used in the growth of nanowires, and their gas sensing properties were studied for ethanol and NO2 as reducing and oxidizing species, respectively. ZnO nanowires grown employing the 6 nm thick layers were the less defective and showed the most stable, repeatable gas sensing properties. Despite ZnO nanowires grown employing the thickest Au layers reached the highest responses under dry conditions, ZnO nanowires grown using the thinnest Au film were more resilient at detecting NO2 in the presence of ambient moisture. The gas sensing results are discussed in light of the defects and the presence of Au impurities in the ZnO nanowires, as revealed by the characterization techniques used, such as X-ray diffraction, field-emission scanning electron microscopy, X-ray photoelectron spectroscopy and photoluminescence spectroscopy. Promising results were obtained by the implementation of ZnO NWs directly grown over alumina substrates for the detection of ethanol and NO2, substantially ameliorating our previously reported results

Photoluminescence of ZnO Nanowires: A Review

One-dimensional ZnO nanostructures (nanowires/nanorods) are attractive materials for applications such as gas sensors, biosensors, solar cells, and photocatalysts. This is due to the relatively easy production process of these kinds of nanostructures with excellent charge carrier transport properties and high crystalline quality. In this work, we review the photoluminescence (PL) properties of single and collective ZnO nanowires and nanorods. As different growth techniques were obtained for the presented samples, a brief review of two popular growth methods, vapor-liquid-solid (VLS) and hydrothermal, is shown. Then, a discussion of the emission process and characteristics of the near-band edge excitonic emission (NBE) and deep-level emission (DLE) bands is presented. Their respective contribution to the total emission of the nanostructure is discussed using the spatial information distribution obtained by scanning transmission electron microscopy−cathodoluminescence (STEM-CL) measurements. Also, the influence of surface effects on the photoluminescence of ZnO nanowires, as well as the temperature dependence, is briefly discussed for both ultraviolet and visible emissions. Finally, we present a discussion of the size reduction effects of the two main photoluminescent bands of ZnO. For a wide emission (near ultra-violet and visible), which has sometimes been attributed to different origins, we present a summary of the different native point defects or trap centers in ZnO as a cause for the different deep-level emission bands

Influence of colloidal Au on the growth of ZnO nanostructures

Vapor-liquid-solid processes allow growing high-quality nanowires from a catalyst. An alternative to the conventional use of catalyst thin films, colloidal nanoparticles offer advantages not only in terms of cost, but also in terms of controlling the location, size, density, and morphology of the grown nanowires. In this work, we report on the influence of different parameters of a colloidal Au nanoparticle suspension on the catalyst-assisted growth of ZnO nanostructures by a vapor-transport method. Modifying colloid parameters such as solvent and concentration, and growth parameters such as temperature, pressure, and Ar gas flow, ZnO nanowires, nanosheets, nanotubes and branched-nanowires can be grown over silica on silicon and alumina substrates. High-resolution transmission electron microscopy reveals the high-crystal quality of the ZnO nanostructures obtained. The photoluminescence results show a predominant emission in the ultraviolet range corresponding to the exciton peak, and a very broad emission band in the visible range related to different defect recombination processes. The growth parameters and mechanisms that control the shape of the ZnO nanostructures are here analyzed and discussed. The ZnO-branched nanowires were grown spontaneously through catalyst migration. Furthermore, the substrate is shown to play a significant role in determining the diameters of the ZnO nanowires by affecting the surface mobility of the metal nanoparticles

Role of aluminum and HMTA in the hydrothermal synthesis of two-dimensional n-doped ZnO nanosheets

This work reports the study of the processes behind the growth of two-dimensional (2D) n-doped ZnO nanostructures on an AlN layer. We have demonstrated that AlN undergoes a slow dissociation process due to the basic controlled environment promoted by the hexamethylenetetramine (HMTA). The Al(OH)4- ions created inhibits the growth along the c-axis, effectively promoting the fast formation of a planar geometry selectively grown on top of the AlN layer. With the use of this promoting layer and a standard hydrothermal method, a selective area growth is observed with micrometric resolution. In addition, by using several advanced characterization techniques such as, X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS/EDX), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL), we observed a resulting doping with aluminum of the ZnO nanostructures, occupying substitutional and interstitial sites, that could lead to new promising applications. These high-quality n-doped ZnO nanosheets (NSs) exhibit strong ultraviolet emission in the 385-405 nm region without broad deep level emission. The piezoelectric nature of these nanostructures has been demonstrated by using piezoresponse atomic force microscope (PFM) and with the support of a piezoelectric test device. Therefore, this low-cost and fast selective-area synthesis of 2D n-doped ZnO NSs can be applicable to other aluminum based materials and paves the way to new promising applications, such as bioelectronic applications, energy generation or self-powered sensin

ZnO Nanowires/N719 dye with different aspect ratio as a possible photoelectrode for Dye-Sensitized Solar Cells

The vapor-liquid-solid (VLS) process was applied to fabricate zinc oxide nanowires (ZnO NWs) with a different aspect ratio (AR), morphological, and optical properties. The ZnO NWs were grown on a system that contains a quartz substrate with transparent conductive oxide (TCO) thin film followed by an Al-doped ZnO (AZO) seed layer; both films were grown by magnetron sputtering at room temperature. It was found that the ZnO NWs presented high crystalline quality and vertical orientation from different structural and morphological characterizations. Also, NWs showed a good density distribution of 69 NWs/μm2 with a different AR that offers their capability to be used as possible photoelectrode (anode) in potential future device applications. The samples optical properties were studied using various techniques such as photoluminescence (PL), absorption, and transmittance before and after sensitization with N719 dye. The results demonstrated that NW with 30 nm diameter had the best characteristics as feasible photoelectrode (anode) (high absorption, minimum recombination, high crystallinity). Also, the present samples optical properties were found to be improved due to the existence of N719 dye and Au nanoparticles on the tip of NWs. NWs grown in this work can be used in different photonic and optoelectronic applications

PdO and PtO doped WS2 boosts NO2 gas sensing characteristics at room temperature

In this work tungsten disulphide nanostructures loaded with platinum-oxide (PtO), or palladium-oxide (PdO) were grown directly onto alumina substrates. This was achieved using a combination of aerosol-assisted chemical vapour deposition (AA-CVD) method with atmospheric pressure CVD technique. At first, tungsten oxide nanowires loaded with either PtO or PdO nanoparticles were successfully co-deposited via AA-CVD followed by sulfurization at 900 °C in the next step. The morphological, structural, and chemical characteristics were investigated using FESEM, TEM, XRD, XPS and Raman spectroscopy. The results confirm the presence of PdO and PtO in the WS2 host matrix. Gas sensing attributes of loaded and pristine WS2 sensors were investigated, at room temperature, towards different analytes (NO2, NH3, H2 etc.). Both pristine and metal-oxide loaded WS2 gas sensors show remarkable responses at room temperature towards NO2 detection. Further, the loaded sensors demonstrated stable, reproducible, ultrasensitive, and enhanced gas sensing response, with a detection limit below 25 ppb. Additionally, the effect of ambient humidity on the sensing response of both loaded and pristine sensors was investigated for NO2 gas. The response of PtO loaded sensor considerably decreased in humid environments, while the response for pristine and PdO loaded sensors increased. However, slightly heating (at 100 °C) the sensors, suppresses the influence of humidity. Finally, the long-term stability of different sensors is investigated, and the results demonstrate high stability with repeatable results after 6 weeks of gas sensing tests. This work exploits an attractive pathway to add functionality in the transition metal dichalcogenide host matrix

Efficient tunable laser operation of Tm:KGd(WO4)2 in the continuous-wave regime at room temperature

Tm:KGd(WO/sub 4/)/sub 2/ is studied as a three-level laser on the /sup 3/F/sub 4/ /spl rarr/ /sup 3/H/sub 6/ transition and a tunable source in the 2-/spl mu/m spectral range, operating at room temperature. An overall tunability extending from 1790 to 2042 nm is achieved with maximum output powers of 400 mW for an absorbed pump power of 1 W. Various doping levels, pump wavelengths and polarization configurations are compared and the advantages of the monoclinic double tungstates over other Tm-hosts are outlined

Nurses' perceptions of aids and obstacles to the provision of optimal end of life care in ICU

Contains fulltext : 172380.pdf (publisher's version ) (Open Access

Làsers d"upconversion: llum làser visible a partir de llum infraroja.

Els làsers aporten solucions a una gran varietat de problemes de la vida diària ..

Espectroscòpia òptica dels ions Tm3+ i Yb3+ en tungstats dobles monoclínics. Un nou làser d'estat sòlid

[cat] L'espectroscòpia òptica dels ions Tm3+ i Yb3+ en tungstats dobles monoclínics és molt important per desenvolupar dispositius làser d'estat sòlid. Els tungstats dobles monoclínics són molt interessants pels avantatges que presenten com a matrius làser. Aquests materials poden créixer i dopar-se amb ions òpticament actius de forma relativament fàcil, i la seva estructura cristal·lina monoclínica provoca una marcada anisotropia òptica, la qual possibilita obtenir radiació làser de forma polaritzada. Els ions Tm3+ i Yb3+ presenten bandes amples i elevades seccions eficaces d'absorció i emissió en els tungstats que faciliten assolir emissions estimulades en un rang ampli de longituds d'ona. A més, els ions Yb3+ sensibilitzen els Tm3+, als quals transfereixen eficientment la seva energia. Els ions Tm3+ i Yb3+ es poden excitar amb certa facilitat amb díodes làser de GaAlAs i InGaAs al voltant de 800 i 980 nm, respectivament. La radiació làser més estudiada dels ions Tm3+ és la que generen al voltant de 2 m, i té aplicacions en els camps de la medicina i dels sensors remots o sistemes radar de l'atmosfera. Aquest treball de recerca s'exposa en cinc capítols. En el primer s'introdueixen els làsers d'estat sòlid i les propietats dels tungstats dobles monoclínics de KGd(WO4)2 i KYb(WO4)2 i dels ions Tm3+ i Yb3+; a continuació es detallen les possibles transferències d'energia entre ions, algunes de les quals donen lloc a mecanismes d'upconversion i es presenten els objectius que es pretenen assolir. En el capítol 2 s'introdueixen les tècniques espectroscòpiques i es descriu l'equipament experimental utilitzat en la caracterització òptica d'aquests materials. Els resultats experimentals s'exposen als capítols 3, 4 i 5. En el capítol 3 s'estudia l'absorció òptica per obtenir l'estructura de nivells energètics i dissenyar les possibles estratègies d'excitació dels ions. Les mesures d'absorció es realitzen sota condicions de llum polaritzada i en funció de la temperatura, a baixa temperatura (6 K) es determina el desdoblament Stark dels nivells excitats dels ions Tm3+. En el capítol 4 s'analitza la luminescència dels ions Tm3+, primer en cristalls de KGd(WO4)2 dopats amb ions Tm3+ i codopats amb ions Tm3+ i Yb3+, i a continuació en cristalls de KYb(WO4)2 dopats amb ions Tm3+. La luminescència s'estudia en funció de la concentració d'ions i de la temperatura, a baixa temperatura (6 K) es determina el desdoblament Stark de l'estat fonamental dels ions Tm3+. En el capítol 5 es fa un breu recull bibliogràfic de l'acció làser al voltant de 2 m dels ions Tm3+ i es presenten els resultats aconseguits en els cristalls de KGd(WO4)2, en els quals es comparen les orientacions amb E//Nm i E//Np, i es posa especial èmfasi en el rang de sintonització assolible. A continuació dels resultats es presenten les conclusions i les referències bibliogràfiques, i finalment s'adjunten les publicacions dels resultats de la tesi doctoral.[eng] Tm3+ -doped laser materials are emerging as very interesting active media for the 2 µm spectral region due to the possibility for diode pumping near 800 nm and their broad tunability. Potential applications lie in the fields of medicine, laser radar and atmosphere monitoring. The monoclinic double tungstates are strongly anisotropic biaxial crystals that allow to obtain polarized laser radiation. Their advantages for highly efficient, low threshold laser operation with diode pumping are well known in the case of Nd3+ and Yb3+ doping but so far only few laser works were devoted to Tm3+ -doped double tungstates. Yb3+ ions are good sensitizers of Tm3+ ions. The advantages of Yb3+ ions are that they absorb in the near-infrared region around 980 nm, where laser diodes work efficiently. In this work we investigate the CW laser characteristics of Tm-doped KGd(WO4)2 on the 3F43H6 transition as a tunable source in the 2 µm spectral range operating at room temperature. An overall tunability extending from 1790 to 2042 nm is achieved with maximum output powers of 400 mW for an absorbed pump power of 1 W. Various doping levels, pump wavelengths and polarization configurations are compared and the advantages of the monoclinic double tungstates over other Tm-hosts are outlined