Synthesis of multi-walled carbon nanotubes by combining hot-wire and dc plasma-enhanced chemical vapor deposition

International audienceMulti-walled carbon nanotubes (MWCNTs) have been grown on 7 nm Ni-coated substrates consisting of crystalline silicon covered with a thin layer (10 nm) of TiN, by combining hot-wire chemical vapor deposition (HWCVD) and direct current plasma-enhanced chemical vapor deposition (dc PECVD), at 620 °C. Acetylene (C2H2) gas is used as the carbon source and ammonia (NH3) and hydrogen (H2) are used either for dilution or etching. The carbon nanotubes range from 20 to 100 nm in diameter and 0.3 to 5 μm in length, depending on growth conditions: plasma intensity, filament current, pressure, C2H2, NH3, H2 flow rates, C2H2/NH3 and C2H2/H2 mass flow ratios. By combining the HWCVD and the dc PECVD processes, uniform growth of oriented MWCNTs was obtained, whereas by using only the HWCVD process, tangled MWCNTs were obtained. By patterning the nickel catalyst, with the use of the HW dc PECVD process, uniform arrays of nanotubes have been grown as well as single free-standing aligned nanotubes, depending on the catalyst patterning (optical lithography or electron-beam lithography). In the latter case, electron field emission from the MWCNTs was obtained with a maximum emission current density of 0.6 A/cm2 for a field of 16 V/μm

Study of electron field emission from arrays of multi-walled carbon nanotubes synthesized by hot-wire dc plasma-enhanced chemical vapor deposition

International audienceMulti-walled carbon nanotubes have been grown on 7 nm Ni-coated substrates consisting of 300 lm thick highly n-doped (1 0 0) sil- icon covered with a diffusion barrier layer (10 nm thick) of SiO2 or TiN, by combining hot-wire chemical vapor deposition and direct current plasma-enhanced chemical vapor deposition at low temperature (around 620 °C). Acetylene gas was used as carbon source and ammonia and hydrogen were used either for dilution or etching. Growth of dense aligned nanotubes could be observed only if the ammonia content was minimized (rv5%). In order to improve the electron field emission properties of the films, different geometrical factors have been taken into account: average length, length/radius ratio and spacing between nanotubes. The nanotube growth rate was controlled by the substrate temperature and the pressure in the reactor, and the nanotube height by the growth time. The nanotube diam- eter was controlled by the catalyst dot volume, and the nanotube spacing was adjusted during the patterning process of the catalyst dots. Using optical lithography, 1 lm Ni dots were obtained and several multi-walled nanotubes with diameter and length in the range 60- 120 nm and rv2.3 lm were grown on each dot. Thus, based on a two-dimensional square lattice with a lattice translation vector of 4 lm, I-V characteristics yielded an onset electric field of 16 V/lm and a maximum emission current density of 40 mA/cm2, due to the large screening effect. Using electron-beam lithography, 100 nm Ni dots were obtained and individual multi-walled nanotubes were grown on each dot. Based on a square lattice with 10 lm translation vector, I-V characteristics gave an onset field of 8 V/lm and a max- imum emission current density of 0.4 A/cm2

Dépôt par plasma en résonance cyclotron électronique d'alliages de silicium pour applications optiques

PALAISEAU-Polytechnique (914772301) / SudocSudocFranceF

Gas phase and surface kinetics in plasma and hot filament-enhanced catalytic chemical vapor deposition of carbon nanostructures

International audienceSimulations of the gas phase chemistry (C2H2/H2) coupled with surface reactions for the catalytic growth of carbon nanostructures (nanotubes/nanofibers), using different activation modes of catalytic chemical vapor deposition (CCVD) process, are presented. Deposits issued from thermal CCVD, hot-filament CCVD, plasma- enhanced CCVD and plasma-enhanced combined with hot-filament CCVD are compared to simulations of the gas phase and surface kinetics. The influence of the activation elements is described in detail. According to these simulations taking into account optical emission spectroscopy data, gas phase composition and linear growth rate of tubular nanostructures are predicted in good agreement with the experimental observations

On the use of soft X-ray STXM for organic-inorganic halide perovskite photovoltaic materials

International audienc

On the use of Soft X-ray STXM for Organic-Inorganic Halide Perovskite Photovoltaic Materials

Perovskite solar cells (PSCs) research is an intense field that could benefit from every available research tool. Synchrotron tools have played an important role in fundamental and applied research for decades. Many synchrotron-based hard X-ray tools are already providing effective feedback to the PSC research community. With several fourth-generation light sources up and running or under development, this contribution will continue to impact every aspect of scientific advancement including PSC research. Arguably, the contribution of soft X-rays in PSC research is relatively limited. In view of the developments in the synchrotron world and the fact that a multimethod approach, combining laboratory-based techniques as well as synchrotron-based techniques, is necessary to provide constructive feedback to the PSC community we present here a collection of arguments and procedures with the aim of highlighting the use of soft X-ray scanning transmission X-ray microscopy (STXM). Some aspects of these arguments are elaborated with STXM investigation of perovskite material formamidinium-methylammonium lead iodide (FA1-xMAxPbI3)

The Role of Oxygen in the Degradation of Methylammonium Lead Trihalide Perovskite Photoactive Layers

International audiencePerovskite materials have already proven their ability to reach photo-electric power conversion efficiencies higher than 22% in appropriate devices. If their instability against time could be solved, they could quickly compete with silicon since they benefit from low-cost manufacturing processes. Our work is dedicated to this stability study using the benefit of X-ray Photoelectron Spectroscopy (XPS) as a main tool, and coupled with XRD and SEM-EDX analysis. Using XPS, it is possible to track the surface (10 nm) changes the perovskite undergoes, in terms of both composition and chemical environments, and is therefore efficient towards understanding the first steps of the degradation process. At first, samples of spin-coated thin-film perovskite without capping on the top (glass/ITO/PEDOT:PSS/MAPI), were aged in different conditions (light with air and vacuum respectively), and analyzed using XPS at different times. The figure below presents the survey spectrum obtained for a fresh sample, and shows that all the expected elements can be identified with this spectrometry method. Starting from an initial known composition, the results obtained reveal interesting changes the material undergoes during the ageing. For example, it was observed that both nitrogen and iodine gradually escape the surface, and also, that metallic lead is observed in the final stages of the degradation process. Then, other currently ongoing experiments are designed to highlight the influence of oxygen and light as previously mentioned , but also to disentangle the influence of the bottom layers on the ageing. This will certainly provide other innovative results on the mechanisms governing the perovskite degradation

The Role of Oxygen in the Degradation of Methylammonium Lead Trihalide Perovskite Photoactive Layers

International audiencePerovskite materials have already proven their ability to reach photo-electric power conversion efficiencies higher than 22% in appropriate devices. If their instability against time could be solved, they could quickly compete with silicon since they benefit from low-cost manufacturing processes. Our work is dedicated to this stability study using the benefit of X-ray Photoelectron Spectroscopy (XPS) as a main tool, and coupled with XRD and SEM-EDX analysis. Using XPS, it is possible to track the surface (10 nm) changes the perovskite undergoes, in terms of both composition and chemical environments, and is therefore efficient towards understanding the first steps of the degradation process. At first, samples of spin-coated thin-film perovskite without capping on the top (glass/ITO/PEDOT:PSS/MAPI), were aged in different conditions (light with air and vacuum respectively), and analyzed using XPS at different times. The figure below presents the survey spectrum obtained for a fresh sample, and shows that all the expected elements can be identified with this spectrometry method. Starting from an initial known composition, the results obtained reveal interesting changes the material undergoes during the ageing. For example, it was observed that both nitrogen and iodine gradually escape the surface, and also, that metallic lead is observed in the final stages of the degradation process. Then, other currently ongoing experiments are designed to highlight the influence of oxygen and light as previously mentioned , but also to disentangle the influence of the bottom layers on the ageing. This will certainly provide other innovative results on the mechanisms governing the perovskite degradation