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 nanocrystals exhibit air-stable plasmonic behavior over periods of more than a year.

To fully deploy the potential of semiconductor nanocrystal films as low-cost electronic materials, a better understanding of the amount of dopants required to make their conductivity metallic is needed. In bulk semiconductors, the critical concentration of electrons at the metal-insulator transition is described by the Mott criterion. Here, we theoretically derive the critical concentration nc for films of heavily doped nanocrystals devoid of ligands at their surface and in direct contact with each other. In the accompanying experiments, we investigate the conduction mechanism in films of phosphorus-doped, ligand-free silicon nanocrystals. At the largest electron concentration achieved in our samples, which is half the predicted nc, we find that the localization length of hopping electrons is close to three times the nanocrystals diameter, indicating that the film approaches the metal-insulator transition.Semiconductor nanocrystals (NCs) have shown great potential in optoelectronics applications such as solar cells [1], light emitting diodes [2], and field-effect transistors [3,4] by virtue of their size-tunable optical and electrical properties [5] and low-cost solution-based processing techniques [6,7]. These applications require conducting NC films and the introduction of extra carriers through doping can enhance the electrical conduction. Several strategies for NC doping have been developed. Remote doping, the use of suitable ligands as donors in the vicinity of NC surface, increased the conductivity of PbSe NC films by 12 orders of magnitude [8]. Electrochemical doping, which tunes the carrier concentration accurately and reversibly, resulted in conducting NC films [9,10]. Lately, stoichiometric control has emerged as a strategy to dope lead chalcogenide NCs [11]. Finally, electronic impurity doping of NCs, originally impeded by synthetic challenges [12], was recently achieved in InAs [13] and CdSe [14] NCs.While many experimental studies have been directed towards increasing the conductivity of NC films, there is still no clear consensus on the fundamental question: what is the condition for the metal-insulator transition (MIT) in NC films [15][16][17]? In a bulk semiconductor, the critical electron concentration n M for the MIT depends on the Bohr radius a B according to the well-known Mott criterion [18] n M a 3 B 0.02, where a B = ε 2 /m * e 2 is the effective Bohr radius (in Gaussian units), ε is the dielectric constant of the semiconductor, and m * is the effective electron mass. It is obvious that a dense film of undoped semiconductor NCs is an insulator, while a film of touching metallic NCs with the same geometry is a conductor. Therefore, the MIT has to occur in semiconductor NC films at some criti-FIG. 1. The origin of the metal-to-insulator transition in semiconductor nanocrystal films. The figure shows the cross section of two nanocrystals in contact through facets with radius ρ. The blue spherical cloud represents an electron wave packet which moves through the contact. Such a compact wave pac...

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.

The doping of semiconductor nanocrystals (NCs), which is vital for the optimization of NC-based devices, remains a significant challenge. While gas-phase plasma approaches have been successful in incorporating dopant atoms into NCs, little is known about their electronic activation. Here, we investigate the electronic properties of doped silicon NC thin films cast from solution by field effect transistor analysis. We find that, analogous to bulk silicon, boron and phosphorus electronically dope Si NC thin films; however, the dopant activation efficiency is only ∼10(-2)-10(-4). We also show that surface doping of Si NCs is an effective way to alter the carrier concentrations in Si NC films.

No abstract

General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. We demonstrate a method for synthesizing free standing silicon nanocrystals in an argon/silane gas mixture by using a remote expanding thermal plasma. Transmission electron microscopy and Raman spectroscopy measurements reveal that the distribution has a bimodal shape consisting of two distinct groups of small and large silicon nanocrystals with sizes in the range 2-10 nm and 50-120 nm, respectively. We also observe that both size distributions are lognormal which is linked with the growth time and transport of nanocrystals in the plasma. Average size control is achieved by tuning the silane flow injected into the vessel. Analyses on morphological features show that nanocrystals are monocrystalline and spherically shaped. These results imply that formation of silicon nanocrystals is based on nucleation, i.e., these large nanocrystals are not the result of coalescence of small nanocrystals. Photoluminescence measurements show that silicon nanocrystals exhibit a broad emission in the visible region peaked at 725 nm. Nanocrystals are produced with ultrahigh throughput of about 100 mg/min and have state of the art properties, such as controlled size distribution, easy handling, and room temperature visible photoluminescence.

We have determined the surface hydride composition of amorphous and crystalline Si nanoparticles (NPs) (3À5 nm) synthesized in a low-temperature SiH 4 /Ar plasma using in situ attenuated total reflection Fourier-transform infrared spectroscopy and H 2 thermal effusion measurements. With increasing power to the plasma source, the particles transition from amorphous to crystalline with a corresponding increase in the fraction of SiH species on the surface. The surface hydride composition indicates that Si NPs synthesized at higher plasma powers crystallize in the gas-phase due to a greater degree of plasma-induced heating, which enhances the desorption rates for SiH 2 and SiH 3 . Furthermore, these Si NPs do not contain any detectable H in the bulk.

Here, we examine ultrafast photoluminescence produced from plasma-grown, colloidal silicon nanocrystals as a function of both particle size and lattice crystallinity. In particular, we quantify the decay time and spectral profiles of nominally fewpicosecond direct-gap emission previously attributed to phononless electron−hole recombination. We find that the high-energy (400− 600 nm, 2−3 eV) photoluminescence component consists of two decay processes with distinct time scales. The fastest photoluminescence exhibits an ∼30 ps decay constant largely independent of emission energy and particle size. Importantly, nearly identical temporal components and blue spectral features appear for amorphous particles. We thus associate high-energy, rapid emission with an amorphous component in all measured samples, as supported by Raman analysis and molecular dynamics simulation. Based on these observations, we advise that the observed dynamics proceed too slowly to originate from intraband carrier thermalization and instead suggest a nonradiative origin associated with the amorphous component.