Oxidation of freestanding silicon nanocrystals probed with electron spin resonance of interfacial dangling bonds

Pereira, Rui N.; Rowe, David J.; Anthony, Rebecca; Kortshagen, Uwe R.

doi:10.1103/physrevb.83.155327

Cited by 66 publications

(120 citation statements)

References 73 publications

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“…In previous work, we found that oxidation follows the Cabrera-Mott mechanism with a characteristic time t m = 14.4 min [40]. Based on this oxidation mechanism, we can investigate the dependence of the localization length on the separation s between NCs.…”

Section: Si4 Oxidationmentioning

confidence: 99%

Metal–insulator transition in films of doped semiconductor nanocrystals

Chen¹,

Reich²,

Kramer³

et al. 2015

Nature Mater

107

134

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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...

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Section: Si4 Oxidationmentioning

confidence: 99%

Metal–insulator transition in films of doped semiconductor nanocrystals

Chen¹,

Reich²,

Kramer³

et al. 2015

Nature Mater

107

134

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“…An extremely low defect density (7×10 9 cm −2 ) is found for Si-NCs grown with additional hydrogen gas injected into the afterglow region of the plasma, hereafter referred to as low initial defect density (LIDD) Si-NCs. We also compare the process of oxidation in air of the LIDD Si-NCs with that of Si-NCs containing an order of magnitude larger initial defect density (MIDD Si-NCs mentioned above), addressed in a previous study, 25 which have been grown without injection of hydrogen gas into afterglow region of the plasma. No change of defect density during the induction period is observed for the LIDD Si-NCs.…”

mentioning

confidence: 99%

“…Si-db reduction upon air exposure was observed for these MIDD Si-NCs during the so-called induction period, before creation of defects initiated by surface oxide shell growth. 25 After complete oxidation the density of defects saturates at a value of 5×10 10 cm −2 . 25 Comparatively, Si-NCs grown from microwave plasma-assisted decomposition of silane 2 display typically one order of magnitude higher density of interfacial Si-dbs (5-7×10 11 cm −2 ) after surface oxidation in air is completed, 19,24 corresponding to 0.25-0.35 defects per NC for Si-NCs with 4 nm in size.…”

mentioning

confidence: 99%

“…25 After complete oxidation the density of defects saturates at a value of 5×10 10 cm −2 . 25 Comparatively, Si-NCs grown from microwave plasma-assisted decomposition of silane 2 display typically one order of magnitude higher density of interfacial Si-dbs (5-7×10 11 cm −2 ) after surface oxidation in air is completed, 19,24 corresponding to 0.25-0.35 defects per NC for Si-NCs with 4 nm in size. This amount of defects may be decreased by up to one order of magnitude only after these Si-NCs are subjected to wet etching in hydrofluoric acid followed by vacuum heating at 200…”

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confidence: 99%

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Freestanding silicon nanocrystals with extremely low defect content

et al. 2012

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The future exploitation of the exceptional properties of freestanding silicon nanocrystals (Si-NCs) in marketable applications relies upon our ability to produce large amounts of defect-free Si-NCs by means of a low-cost method. Here, we demonstrate that Si-NCs fabricated by scalable RF plasmaassisted decomposition of silane with additional hydrogen gas injected into the afterglow region of the plasma exhibit immediately after synthesis the lowest reported defect density, corresponding to a value of only about 0.002-0.005 defects per NC for Si-NCs with 4 nm in size. In addition, the virtually perfect hydrogen termination of these Si-NCs yields an enhanced resistance against natural oxidation in comparison to Si-NCs with nearly one order of magnitude larger initial defect density.

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“…It is well-recognized that the oxidation of silicon nanocrystals usually follows the Cabrera-Mott mechanism, 29,30 as supported by previous investigations. [31][32][33] This mechanism describes how a Si crystal surface is oxidized by the simultaneous presence of water and oxygen molecules in the ambient air. Specifically, the polar water molecules approach the surface silanol groups preferentially and aid in the cleavage of Si-Si bonds adjacent to the silanol groups.…”

Section: -14mentioning

confidence: 99%

Non-wettable, Oxidation-Stable, Brightly Luminescent, Perfluorodecyl-Capped Silicon Nanocrystal Film

et al. 2014

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Here we describe for the first time the synthesis of colloidally stable, brightly luminescent perfluorodecylcapped silicon nanocrystals and compare the properties of solutions and films made from them with those of their perhydrodecyl-capped relatives. The perfluorodecyl capping group compared to the perhydrodecyl capping group yields superior hydrophobicity and much greater resistance to air oxidation, the enhanced electron-withdrawing character induces blue shifts in the wavelength of photoluminescence, and the lowerfrequency carbon-fluorine stretching modes disfavor non-radiative relaxation pathways and boost the absolute photoluminescence quantum yield. Together these attributes bode well for advanced materials and biomedical applications founded upon perfluorodecyl-protected silicon nanocrystals. Keywordscapped, perfluorodecyl, luminescent, brightly, stable, oxidation, wettable, nanocrystal, non, silicon, film Disciplines Engineering | Physical Sciences and Mathematics Publication DetailsQian, C., Sun, W., Wang, L., Chen, C., Liao, K., Wang, W., Jia, J., Hatton, B., Casillas, G., Kurylowicz, M., Yip, Supporting Information PlaceholderABSTRACT: Here we describe for the first time the synthesis of colloidally-stable, brightly-luminescent perfluorodecylcapped silicon nanocrystals and compare the properties of solutions and films with the perhydrodecyl-capped relative. The perfluorodecyl capping group compared to the perhydrodecyl capping group yields superior hydrophobicity and much greater resistance to air oxidation, the enhanced electron withdrawing character induces blue shifts in the wavelength of photoluminescence and the lower frequency carbon-fluorine stretching modes disfavor non-radiative relaxation pathways and boost the absolute photoluminescence quantum yield. Together these attributes bode well for advanced materials and biomedical applications founded upon perfluorodecyl-protected silicon nanocrystals.The burgeoning research activity on new kinds of nanostructured silicon, made from one of the most abundant and green materials on earth, is striking.

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Oxidation of freestanding silicon nanocrystals probed with electron spin resonance of interfacial dangling bonds

Cited by 66 publications

References 73 publications

Metal–insulator transition in films of doped semiconductor nanocrystals

Metal–insulator transition in films of doped semiconductor nanocrystals

Freestanding silicon nanocrystals with extremely low defect content

Non-wettable, Oxidation-Stable, Brightly Luminescent, Perfluorodecyl-Capped Silicon Nanocrystal Film

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