In 1994 Bhargava et al. [1] reported that nanocrystalline ZnS:Mn2+ can yield both high luminescen... more In 1994 Bhargava et al. [1] reported that nanocrystalline ZnS:Mn2+ can yield both high luminescence efficiencies and a decay time shortening from ms to ns at the same time. These spectacular results suggested that doped semiconductor nanoparticles form a new class of luminescent materials with a wide range of applications in, for example, displays, sensors and lasers...
... Each paper reports on one particular passivator (eg, acrylic acid (AA), methacrylic acid (MA)... more ... Each paper reports on one particular passivator (eg, acrylic acid (AA), methacrylic acid (MA), poly(methyl methacrylate) (PMMA), dodecylbenzenesulfonic acid ... of the various samples were calculated using the lamp phosphor BaMgAl 10 O 17 :Eu 2+ (BAM, commercial phosphor ...
Five years ago it was reported that nanocrystalline ZnS : Mn> can yield both high luminescence e$... more Five years ago it was reported that nanocrystalline ZnS : Mn> can yield both high luminescence e$ciencies and a spectacular lifetime shortening, which suggested 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 Mn> emission of nanocrystalline ZnS : Mn> does in fact not show a spectacular shortening of the decay time upon decreasing particle size. The luminescence of nanocrystalline ZnS : Mn> indeed shows a short decay time component (&100 ns), but also a long (ms) decay time is observed. The short decay time is ascribed to a defect-related emission of ZnS, and is not from the decay of the T }A transition of the Mn> impurity. The T }A transition of the Mn> 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 e$ciency with a short (ns) decay time.
Recently, Yu and co-workers reported the observation of hot luminescence for Mn 2+ in 3.6 nm ZnS ... more Recently, Yu and co-workers reported the observation of hot luminescence for Mn 2+ in 3.6 nm ZnS nanocrystals, implying that the phonon relaxation rates in nanocrystals were B10 6 times lower than that in bulk materials. In order to verify this unexpected observation, we investigated the time-resolved spectroscopy of 4.5 nm ZnS:Mn 2+ nanocrystals. Samples with two different Mn 2+ concentrations were studied. In contradiction with the results reported by Yu and co-workers, no evidence of Mn 2+ hot luminescence was found, regardless of the Mn 2+ concentration, temperature (4.2-300 K), time delay (0.1-1000 ms), gate width (2-3000 ms), or excitation density (40-700 mJ/cm 2 ). r
Room temperature time-resolved luminescence measurements on single CdSe/ZnS quantum dots (QDs) ar... more Room temperature time-resolved luminescence measurements on single CdSe/ZnS quantum dots (QDs) are presented. Luminescence spectra were recorded over time periods of up to 30 min with a time resolution down to 6 ms. A clear 30 ± 40 nm blue shift in the emission wavelength is observed for QDs in ambient air, before the luminescence stops after about 2 ± 3 min due to photobleaching. The blue shift is absent in a nitrogen atmosphere and photobleaching occurs after much longer times, 10 ± 15 min. These observations are explained by photoinduced oxidation. During illumination in the presence of oxygen the surface of CdSe is oxidized, which effectively results in shrinkage of the CdSe core diameter by almost 1 nm and, consequently, in a blue shift. This also influences the blinking behavior. The faster fading of the luminescence in air suggests that photoinduced oxidation results in the formation of nonradiative recombination centers at the CdSe/ CdSeO x interface. In a nitrogen atmosphere photoinduced oxidation is prevented by the absence of oxygen. A surprising observation is a higher initial light output for CdSe/ZnS QDs in air. We explain this higher light output by a fast reduction of the lifetime of the long-lived defect states of CdSe/ZnS QDs by oxygen.
Recently, it has been reported that doped semiconductor nanoparticles can yield both high lumines... more Recently, 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 ...
Optimized chemical vapor deposition processes for single-walled carbon nanotube (SWCNT) can lead ... more 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.
ABSTRACT This work presents an improved thin film approximation to extract the optical conductivi... more ABSTRACT This work presents an improved thin film approximation to extract the optical conductivity from infrared transmittance in a simple yet accurate way. This approximation takes into account the incoherent reflections from the backside of the substrate. These reflections are shown to have a significant effect on the extracted optical conductivity and hence on derived parameters as carrier mobility and density. By excluding the backside reflections, the error for these parameters for typical chemical vapor deposited (CVD) graphene on a silicon substrate can be as high as 17% and 45% for the carrier mobility and density, respectively. For the mid- and near-infrared, the approximation can be simplified such that the real part of the optical conductivity is extracted without the need for a parameterization of the optical conductivity. This direct extraction is shown for Fourier transform infrared (FTIR) transmittance measurements of CVD graphene on silicon in the photon energy range of 370–7000 cm−1. From the real part of the optical conductivity, the carrier density, mobility, and number of graphene layers are determined but also residue, originating from the graphene transfer, is detected. FTIR transmittance analyzed with the improved thin film approximation is shown to be a non-invasive, easy, and accurate measurement and analysis method for assessing the quality of graphene and can be used for other 2-D materials.
Bimetallic core/shell nanoparticles (NPs) are the subject of intense research due to their unique... more Bimetallic core/shell nanoparticles (NPs) are the subject of intense research due to their unique electronic, optical and catalytic properties. Accurate and independent control over the dimensions of both core and shell would allow for unprecedented catalytic performance. Here, we demonstrate that both core and shell dimensions of Pd/Pt core/shell nanoparticles (NPs) supported on Al2O3 substrates can be controlled at the sub-nanometer level by using a novel strategy based on atomic layer deposition (ALD). From the results it is derived that the main conditions for accurate dimension control of these core/shell NPs are: (i) a difference in surface energy between the deposited core metal and the substrate to obtain island growth; (ii) a process yielding linear growth of the NP cores with ALD cycles to obtain monodispersed NPs with a narrow size distribution; (iii) a selective ALD process for the shell metal yielding a linearly increasing thickness to obtain controllable shell growth e...
Atomic layer deposition (ALD) is a method that allows for the deposition of thin films with atomi... more Atomic layer deposition (ALD) is a method that allows for the deposition of thin films with atomic level control of the thickness and an excellent conformality on 3-dimensional surfaces. In recent years, ALD has been implemented in many applications in microelectronics, for which often a patterned film instead of full area coverage is required. This article reviews several approaches for the patterning of ALD-grown films. In addition to conventional methods relying on etching, there has been much interest in nanopatterning by area-selective ALD. Area-selective approaches can eliminate compatibility issues associated with the use of etchants, lift-off chemicals, or resist films. Moreover, the use of ALD as an enabling technology in advanced nanopatterning methods such as spacer defined double patterning or block copolymer lithography is discussed, as well as the application of selective ALD in self-aligned fabrication schemes.
ABSTRACT In summary, results are presented on the effects of Lg and CL scaling in CNTFETs. To con... more ABSTRACT In summary, results are presented on the effects of Lg and CL scaling in CNTFETs. To consider CNTFETs for a high performance technology, any limitations related to their aggressive scaling must be understood. Devices with excellent and reproducible performance were fabricated with Lg and CL as small as 15 and 20 nm, respectively-projected achievable pitch of 45 nm, which would be suitable beyond the 8 nm technology node. In fact, the smallest CNTFET to date is demonstrated having CL ≈ Lg ≈ 20 nm with superb performance.
In 1994 Bhargava et al. [1] reported that nanocrystalline ZnS:Mn2+ can yield both high luminescen... more In 1994 Bhargava et al. [1] reported that nanocrystalline ZnS:Mn2+ can yield both high luminescence efficiencies and a decay time shortening from ms to ns at the same time. These spectacular results suggested that doped semiconductor nanoparticles form a new class of luminescent materials with a wide range of applications in, for example, displays, sensors and lasers...
... Each paper reports on one particular passivator (eg, acrylic acid (AA), methacrylic acid (MA)... more ... Each paper reports on one particular passivator (eg, acrylic acid (AA), methacrylic acid (MA), poly(methyl methacrylate) (PMMA), dodecylbenzenesulfonic acid ... of the various samples were calculated using the lamp phosphor BaMgAl 10 O 17 :Eu 2+ (BAM, commercial phosphor ...
Five years ago it was reported that nanocrystalline ZnS : Mn> can yield both high luminescence e$... more Five years ago it was reported that nanocrystalline ZnS : Mn> can yield both high luminescence e$ciencies and a spectacular lifetime shortening, which suggested that doped semiconductor nanoparticles form a new class of luminescent materials for various applications. From lifetime measurements and time-resolved spectroscopy we conclude that the Mn> emission of nanocrystalline ZnS : Mn> does in fact not show a spectacular shortening of the decay time upon decreasing particle size. The luminescence of nanocrystalline ZnS : Mn> indeed shows a short decay time component (&100 ns), but also a long (ms) decay time is observed. The short decay time is ascribed to a defect-related emission of ZnS, and is not from the decay of the T }A transition of the Mn> impurity. The T }A transition of the Mn> has a &normal' decay of about 1.9 ms. Based on our observations, we conclude that doped semiconductor nanoparticles do not form a new class of luminescent materials, combining a high e$ciency with a short (ns) decay time.
Recently, Yu and co-workers reported the observation of hot luminescence for Mn 2+ in 3.6 nm ZnS ... more Recently, Yu and co-workers reported the observation of hot luminescence for Mn 2+ in 3.6 nm ZnS nanocrystals, implying that the phonon relaxation rates in nanocrystals were B10 6 times lower than that in bulk materials. In order to verify this unexpected observation, we investigated the time-resolved spectroscopy of 4.5 nm ZnS:Mn 2+ nanocrystals. Samples with two different Mn 2+ concentrations were studied. In contradiction with the results reported by Yu and co-workers, no evidence of Mn 2+ hot luminescence was found, regardless of the Mn 2+ concentration, temperature (4.2-300 K), time delay (0.1-1000 ms), gate width (2-3000 ms), or excitation density (40-700 mJ/cm 2 ). r
Room temperature time-resolved luminescence measurements on single CdSe/ZnS quantum dots (QDs) ar... more Room temperature time-resolved luminescence measurements on single CdSe/ZnS quantum dots (QDs) are presented. Luminescence spectra were recorded over time periods of up to 30 min with a time resolution down to 6 ms. A clear 30 ± 40 nm blue shift in the emission wavelength is observed for QDs in ambient air, before the luminescence stops after about 2 ± 3 min due to photobleaching. The blue shift is absent in a nitrogen atmosphere and photobleaching occurs after much longer times, 10 ± 15 min. These observations are explained by photoinduced oxidation. During illumination in the presence of oxygen the surface of CdSe is oxidized, which effectively results in shrinkage of the CdSe core diameter by almost 1 nm and, consequently, in a blue shift. This also influences the blinking behavior. The faster fading of the luminescence in air suggests that photoinduced oxidation results in the formation of nonradiative recombination centers at the CdSe/ CdSeO x interface. In a nitrogen atmosphere photoinduced oxidation is prevented by the absence of oxygen. A surprising observation is a higher initial light output for CdSe/ZnS QDs in air. We explain this higher light output by a fast reduction of the lifetime of the long-lived defect states of CdSe/ZnS QDs by oxygen.
Recently, it has been reported that doped semiconductor nanoparticles can yield both high lumines... more Recently, 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 ...
Optimized chemical vapor deposition processes for single-walled carbon nanotube (SWCNT) can lead ... more 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.
ABSTRACT This work presents an improved thin film approximation to extract the optical conductivi... more ABSTRACT This work presents an improved thin film approximation to extract the optical conductivity from infrared transmittance in a simple yet accurate way. This approximation takes into account the incoherent reflections from the backside of the substrate. These reflections are shown to have a significant effect on the extracted optical conductivity and hence on derived parameters as carrier mobility and density. By excluding the backside reflections, the error for these parameters for typical chemical vapor deposited (CVD) graphene on a silicon substrate can be as high as 17% and 45% for the carrier mobility and density, respectively. For the mid- and near-infrared, the approximation can be simplified such that the real part of the optical conductivity is extracted without the need for a parameterization of the optical conductivity. This direct extraction is shown for Fourier transform infrared (FTIR) transmittance measurements of CVD graphene on silicon in the photon energy range of 370–7000 cm−1. From the real part of the optical conductivity, the carrier density, mobility, and number of graphene layers are determined but also residue, originating from the graphene transfer, is detected. FTIR transmittance analyzed with the improved thin film approximation is shown to be a non-invasive, easy, and accurate measurement and analysis method for assessing the quality of graphene and can be used for other 2-D materials.
Bimetallic core/shell nanoparticles (NPs) are the subject of intense research due to their unique... more Bimetallic core/shell nanoparticles (NPs) are the subject of intense research due to their unique electronic, optical and catalytic properties. Accurate and independent control over the dimensions of both core and shell would allow for unprecedented catalytic performance. Here, we demonstrate that both core and shell dimensions of Pd/Pt core/shell nanoparticles (NPs) supported on Al2O3 substrates can be controlled at the sub-nanometer level by using a novel strategy based on atomic layer deposition (ALD). From the results it is derived that the main conditions for accurate dimension control of these core/shell NPs are: (i) a difference in surface energy between the deposited core metal and the substrate to obtain island growth; (ii) a process yielding linear growth of the NP cores with ALD cycles to obtain monodispersed NPs with a narrow size distribution; (iii) a selective ALD process for the shell metal yielding a linearly increasing thickness to obtain controllable shell growth e...
Atomic layer deposition (ALD) is a method that allows for the deposition of thin films with atomi... more Atomic layer deposition (ALD) is a method that allows for the deposition of thin films with atomic level control of the thickness and an excellent conformality on 3-dimensional surfaces. In recent years, ALD has been implemented in many applications in microelectronics, for which often a patterned film instead of full area coverage is required. This article reviews several approaches for the patterning of ALD-grown films. In addition to conventional methods relying on etching, there has been much interest in nanopatterning by area-selective ALD. Area-selective approaches can eliminate compatibility issues associated with the use of etchants, lift-off chemicals, or resist films. Moreover, the use of ALD as an enabling technology in advanced nanopatterning methods such as spacer defined double patterning or block copolymer lithography is discussed, as well as the application of selective ALD in self-aligned fabrication schemes.
ABSTRACT In summary, results are presented on the effects of Lg and CL scaling in CNTFETs. To con... more ABSTRACT In summary, results are presented on the effects of Lg and CL scaling in CNTFETs. To consider CNTFETs for a high performance technology, any limitations related to their aggressive scaling must be understood. Devices with excellent and reproducible performance were fabricated with Lg and CL as small as 15 and 20 nm, respectively-projected achievable pitch of 45 nm, which would be suitable beyond the 8 nm technology node. In fact, the smallest CNTFET to date is demonstrated having CL ≈ Lg ≈ 20 nm with superb performance.
Uploads
Papers by Ageeth Bol