Showing papers by "Uwe Kortshagen published in 2015"
TL;DR: It is demonstrated that phosphorus-doped silicon nanocrystals exhibit a plasmon resonance immediately after their synthesis but may lose their plasMonic response with oxidation, and boron-dopes exhibit air-stable plAsmonic behavior over periods of more than a year.
Abstract: Degenerately doped silicon nanocrystals are appealing plasmonic materials due to silicon’s low cost and low toxicity. While surface plasmonic resonances of boron-doped and phosphorus-doped silicon nanocrystals were recently observed, there currently is poor understanding of the effect of surface conditions on their plasmonic behavior. Here, we demonstrate that phosphorus-doped silicon nanocrystals exhibit a plasmon resonance immediately after their synthesis but may lose their plasmonic response with oxidation. In contrast, boron-doped nanocrystals initially do not exhibit plasmonic response but become plasmonically active through postsynthesis oxidation or annealing. We interpret these results in terms of substitutional doping being the dominant doping mechanism for phosphorus-doped silicon nanocrystals, with oxidation-induced defects trapping free electrons. The behavior of boron-doped silicon nanocrystals is more consistent with a strong contribution of surface doping. Importantly, boron-doped silicon ...
138 citations
TL;DR: A nanosecond transient yellow emission band at 590 nm is interpreted as a critical limit of the wavelength tunability in colloidal silicon nanocrystals, confirming that the PL originating from the quantum confined core states can only exist in the red/near infrared with energy below 2.1 eV.
Abstract: Luminescent silicon nanocrystals (Si NCs) have attracted tremendous research interest. Their size dependent photoluminescence (PL) shows great promise in various optoelectronic and biomedical applications and devices. However, it remains unclear why the exciton emission is limited to energy below 2.1 eV, no matter how small the nanocrystal is. Here we interpret a nanosecond transient yellow emission band at 590 nm (2.1 eV) as a critical limit of the wavelength tunability in colloidal silicon nanocrystals. In the “large size” regime (d > ~3 nm), quantum confinement dominantly determines the PL wavelength and thus the PL peak blue shifts upon decreasing the Si NC size. In the “small size” regime (d < ~2 nm) the effect of the yellow band overwhelms the effect of quantum confinement with distinctly increased nonradiative trapping. As a consequence, the photoluminescence peak does not exhibit any additional blue shift and the quantum yield drops abruptly with further decreasing the size of the Si NCs. This finding confirms that the PL originating from the quantum confined core states can only exist in the red/near infrared with energy below 2.1 eV; while the blue/green PL originates from surface related states and exhibits nanosecond transition.
67 citations
TL;DR: It is proposed that doping of central cores and heavy doping of small NCs are achievable via nonthermal plasma synthesis, because chemical potential differences between dopant and host atoms-which hinder dopant incorporation in colloidal synthesis-are irrelevant when NC nucleation and growth proceed via irreversible interactions among highly reactive gas-phase ions and radicals and ligand-free NC surfaces.
Abstract: Metal oxide semiconductor nanocrystals (NCs) exhibit localized surface plasmon resonances (LSPRs) tunable within the infrared (IR) region of the electromagnetic spectrum by vacancy or impurity doping. Although a variety of these NCs have been produced using colloidal synthesis methods, incorporation and activation of dopants in the liquid phase has often been challenging. Herein, using Al-doped ZnO (AZO) NCs as an example, we demonstrate the potential of nonthermal plasma synthesis as an alternative strategy for the production of doped metal oxide NCs. Exploiting unique, thoroughly nonequilibrium synthesis conditions, we obtain NCs in which dopants are not segregated to the NC surfaces and local doping levels are high near the NC centers. Thus, we achieve overall doping levels as high as 2 × 10(20) cm(-3) in NCs with diameters ranging from 12.6 to 3.6 nm, and for the first time experimentally demonstrate a clear quantum confinement blue shift of the LSPR energy in vacancy- and impurity-doped semiconductor NCs. We propose that doping of central cores and heavy doping of small NCs are achievable via nonthermal plasma synthesis, because chemical potential differences between dopant and host atoms-which hinder dopant incorporation in colloidal synthesis-are irrelevant when NC nucleation and growth proceed via irreversible interactions among highly reactive gas-phase ions and radicals and ligand-free NC surfaces. We explore how the distinctive nucleation and growth kinetics occurring in the plasma influences dopant distribution and activation, defect structure, and impurity phase formation.
56 citations
TL;DR: In this paper, the second derivative of the Langmuir probe current-voltage characteristics was measured in a capacitively coupled argon-silane dusty plasma using a fast-scanning and shielded Langmauir probe.
Abstract: Langmuir probe measurements in dusty plasmas is a challenge because particle and film deposition on the probe leads to contamination and distortion of the current–voltage characteristics. This problem is particularly acute while determining the electron energy probability function (EEPF) from the second derivative of the Langmuir probe current–voltage characteristics. Here, we present reliable EEPF measurements in a capacitively coupled argon–silane dusty plasma using a fast-scanning and shielded Langmuir probe. A solenoid-actuated shield covered the probe and the probe was exposed to the plasma only for short periods of time (less than 6 s) when the current–voltage characteristics were recorded during rapid voltage scans. This approach minimized probe surface contamination. In presence of dust (silicon nanoparticles) the electron density decreased and the electron temperature increased in comparison to a pristine argon plasma. While the population of lower energy electrons decreased in presence of dust, the high energy tail region overlapped throughout the experiment. Langmuir probe measurements were complemented with ion density measurements using a capacitive probe and ex situ examination of particles using electron microscopy.
47 citations
TL;DR: In this article, a heating and charging model for silicon nanoparticles during their synthesis in plasmas maintained over a wide range of pressures (10 − 105 Pa) was presented.
Abstract: While well-defined high quality semiconductor nanocrystals have been synthesized successfully in low pressure nonthermal plasmas, moving the field of plasma nanoparticle synthesis to atmospheric pressures is important for lowering its cost and making the process attractive for some industrial applications. Here we present a heating and charging model for silicon nanoparticles during their synthesis in plasmas maintained over a wide range of pressures (10 − 105 Pa). We consider three collisionality regimes and determine the dominant contribution of each regime to heating and charging of nanoparticles under various plasma conditions. For plasmas maintained at atmospheric pressures we find that the ion current is mainly due to the collisional hydrodynamic contribution. Based on the model, we predict that the formation of nanocrystals at atmospheric pressure requires significantly higher plasma densities than those at low pressures. Strong nanoparticle cooling at atmospheric pressures necessitates high ion densities to reach temperatures required for crystallization of nanoparticles. Using experimentally determined plasma properties from the literature we estimate the nanoparticle temperature that can be achieved during synthesis at atmospheric pressures and predict that temperatures well above those required for crystallization can be achieved. Based on these results we suggest design principles for nanocrystal synthesis at atmospheric pressures.
37 citations
TL;DR: It is suggested that rapid exciton transfer from "bright" nanocrystals to surface trap states in nearest-neighbors can efficiently fill such traps and enhance the stability of emission by promoting the radiative recombination of slowly diffusing excited electrons.
Abstract: Close-packed assemblies of ligand-passivated colloidal nanocrystals can exhibit enhanced photoluminescent stability, but the origin of this effect is unclear. Here, we use experiment, simulation, and ab initio computation to examine the influence of interparticle interactions on the photoluminescent stability of silicon nanocrystal aggregates. The time-dependent photoluminescence emitted by structures ranging in size from a single quantum dot to agglomerates of more than a thousand is compared with Monte Carlo simulations of noninteracting ensembles using measured single-particle blinking data as input. In contrast to the behavior typically exhibited by the metal chalcogenides, the measured photoluminescent stability shows an enhancement with respect to the noninteracting scenario with increasing aggregate size. We model this behavior using time-dependent density functional theory calculations of energy transfer between neighboring nanocrystals as a function of nanocrystal size, separation, and the presen...
35 citations
TL;DR: In this article, a combined experimental and theoretical study of the relationship between the surface structure of silicon nanocrystals synthesized in a nonthermal plasma reactor and their photoluminescence (PL) yields was performed.
Abstract: We report a combined experimental and theoretical study of the relationship between the surface structure of silicon nanocrystals synthesized in a nonthermal plasma reactor and their photoluminescence (PL) yields. Upon heating to 160 °C, a significant change in the SiH stretch region of the vibrational spectrum is observed indicating a decrease in surface SiH3 groups, which correlates with an increase in the PL yield. Effusion of SiHx and Si2H2x from the material is detected by residual gas analysis upon heating to temperatures below 200 °C, suggesting a weakly bound species. Analysis of electron paramagnetic resonance spectra before and after heating points to a small reduction in the density of dangling bonds upon heating but this reduction does not correlate with the increase in PL yield. Electronic structure calculations indicate that SiH3– groups may hypervalently bond to fully coordinated surface silicon atoms, resulting in a relatively weak (0.70 eV) bond that is consistent with the experimentally ...
23 citations
TL;DR: In this paper, the synthesis of ZnS, Cu2S and SnS nanocrystals from metal-sulfide precursors was studied in a nonthermal plasmas.
Abstract: Nanocrystal synthesis in nonthermal plasmas has been focused on elemental group IV semiconductors such as Si and Ge. In contrast, very little is known about plasma synthesis of compound nanocrystals and the time is ripe to extend this synthesis approach to nanocrystals comprised of two or more elements such as metal sulfides, oxides and nitrides. Towards this end, we studied, in an argon–sulfur plasma, the synthesis of ZnS, Cu2S and SnS nanocrystals from metalorganic precursors diethyl Zn(II), hexafluoroacetylacetonate Cu(I) vinyltrimethylsilane, and tetrakis(dimethylamido) Sn(IV), respectively. In situ optical emission spectroscopy was used to observe changes in relative concentrations of various plasma species during synthesis, while ex situ material characterization was used to examine the crystal structure, elemental composition and optical absorption of these nanocrystals. For a constant metalorganic vapor feed rate, the elemental composition of the nanocrystals was found to be independent of the sulfur flow rate into the plasma, above a small threshold value. At constant sulfur flow rate, the nanocrystal composition depended on the metalorganic vapor feed rate. Specifically, the ensemble metal atomic fraction in the nanocrystals was found to increase with increasing metalorganic vapor flow rates, resulting in more metal-rich crystal phases. The metalorganic feed rate can be used to control the composition and crystal phase of the metal-sulfide nanocrystals synthesized using this plasma process.
20 citations
TL;DR: The photostability of luminescent silicon quantum dots is critical for optoelectronic and photovoltaic applications as mentioned in this paper, but their optical properties degrade with UV exposure, and a 20% reduction in quantum yield was observed within 4 h of UV irradiation.
Abstract: The photostability of luminescent silicon quantum dots is critical for optoelectronic and photovoltaic applications. While nanocrystals synthesized in a nonthermal plasma and thermally-hydrosilylated with dodecyl groups exhibit quantum yields exceeding 60%, their optical properties degrade with UV exposure. A 20% (absolute) reduction in quantum yield was observed within 4 h of UV irradiation. The origin of instability was identified to stem from unpaired electrons generated at the nanocrystal surface as a result of the breaking of silicon hydride bonds. Recovery of the nanocrystals' quantum yield can be achieved by passivating the dangling bonds generated during photobleaching. Moreover, no degradation in optical properties was observed with further UV irradiation, indicating that photostable silicon nanocrystals were synthesized.
15 citations
TL;DR: In this paper, a model is developed to fit the photoluminescence (PL) spectrum based on spontaneous emission and the size distribution of the quantum dots (QDs).
Abstract: Narrow size distribution of quantum dots (QDs) is needed for their application in photovoltaics but collection of such information is difficult. Many experiments have shown the photoluminescence (PL) spectrum of Si QDs will broaden and peak position is size dependent. However, there is still lack of quantitative analysis of such phenomenon. In this paper, a model is developed to fit the PL spectrum based on spontaneous emission and the size distribution of the QDs. With this model, we can quantitatively analyse the QD size and its distribution using the PL spectra only, saving the need of time consuming and destructive characterization methods such as transmission electron microscopy (TEM). The optical bandgap can be extracted naturally from this PL model. The size and distribution of the QD which are obtained by fitting the PL spectra are then confirmed by measurements using TEM and XRD.
13 citations
TL;DR: In this article, the amplitude of the atomic vibration (phonon) at the real physical boundary of NPs is proportional to e � α/2, which indicates that the first order optical phonon is relatively largeratthe edges for smaller Si nanostructures despite of their stronger phonon confinement weighed by α/d 2.
Abstract: Asize-dependentevolutionofphononconfinement isrevealedinSinanoparticles (NPs)viaRaman spectroscopy.Byintroducing a variable confinement factor, α, into a well-known phenomenological phonon confinement model (PCM) developed by Richter et al., acceptable fits are achieved to downshifted and asymmetrically broadened Raman spectra of Si NPs with different diameters, d, from 2.4nm to 6.3nm. A comparative study using Raman spectra of colloidal Si NPs, for the first time, shows an apparent positive linear correlation between α and the Si NP size. Based on the PCM, the amplitude of the atomic vibration (phonon) at the real physical boundary of NPs is proportional to e � α/2 , which indicates that the amplitude of the first order optical phonon is relatively largeratthe edges for smaller Si nanostructures despite oftheir strongerphonon confinement weighed by α/d 2 . Copyright © 2015 John Wiley & Sons, Ltd. Additional supporting information may be found in the online version of this article at the publisher’s web site.
Patent•
20 Apr 2015TL;DR: Group IV nanocrystals having a surface substantially free of oxygen, and methods of making such Group IV nanoparticles, are disclosed in this paper, where they can advantageously be used to prepare nanocrystal inks and films.
Abstract: Group IV nanocrystals having a surface substantially free of oxygen, and methods of making such Group IV nanocrystals, are disclosed herein. Group IV nanocrystals having a surface substantially free of oxygen can advantageously be used to prepare nanocrystal inks and films.
TL;DR: In this article, photonic crystals of plasmonic/excitonic semiconductor nanocrystals (NCs) were assembled from non-thermal plasma-synthesized boron (B)-doped silicon (Si) NCs.
Abstract: In this work, photonic crystals of plasmonic/excitonic semiconductor nanocrystals (NCs) were assembled from non-thermal plasma-synthesized boron (B)-doped silicon (Si) NCs. The photonic crystals form an inverse opal structure with larger refractive index than the conventional crystals made from silica nanoparticles and are aimed at controlling light propagation via excitonic and plasmonic absorption of the B-doped Si NC as well as the photonic band gap of the photonic crystal. Furthermore, we demonstrate self-assembly of mesoscopic photonic crystal particles consisting of B-doped Si NCs with well-defined inverse opal structure via simple aerosol processing.
Patent•
14 Apr 2015
TL;DR: In this article, a gas phase process for the production of conductive metal oxide films is described, in which a mixture of a metal precursor in the gaseous state and an oxygen-containing compound in the gas phase is converted into particles in the aerosol state.
Abstract: The present invention is in the field of gas phase processes for the production of conductive metal oxide films. In particular the present invention relates to a process for producing a conductive metal oxide film comprising (a) preparing a mixture comprising a metal precursor in the gaseous state and an oxygen-containing compound in the gaseous state, (b) converting the mixture into particles which are in the aerosol state, (c) depositing the particles from the aerosol state onto a substrate, (d) coating the particles deposited on the substrate with a metal oxide.