Optical band gap of semiconductive type II Si clathrate purified by centrifugation

Himeno, Roto; Ohashi, Fumitaka; Kume, Tetsuji; Asai, Erika; Ban, Takayuki; Suzuki, T.; Iida, Tamio; Habuchi, Hitoe; Tsutsumi, Yasuo; Natsuhara, H.; Nonomura, Shuichi

doi:10.1016/j.jnoncrysol.2011.12.064

Cited by 15 publications

(14 citation statements)

References 15 publications

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“…The absorption onset of the Si clathrate occurred at 2.2 eV, close to prior literature reports of band gaps between 1.8 and 2.0 eV. 15,28 Similarly, the absorption onset for the Ge clathrate was determined to be 0.65 eV, close to the previously reported value of 0.6 eV. 20 Within the alloy system, absorption onsets for samples with y ¼ 0.1 and y ¼ 0.75 compositions were determined to be 1.7 eV and 1.2 eV, respectively.…”

Section: Optical Propertiessupporting

confidence: 85%

“…At these doping levels, signicant free carrier absorption can be expected, and has been previously observed in type II Si clathrates. 8,28,29 The free carrier absorption below the band edge can be t using the Drude model. 8 However, performing a t to the Drude model requires estimation of the carrier lifetime, carrier concentration, effective mass, and index of refraction, all of which are unknown for the type II clathrates.…”

Section: Optical Propertiesmentioning

confidence: 99%

“…15 Slightly lower values of 1.6-1.8 eV have been reported for a wide range of Na occupancies (x ¼ 0.6-15), and an increase in free carrier absorption at low energies (0.5-1.4 eV) was observed with increasing Na occupancy. 28,29 The difficultly of synthesizing nearly guest-free Ge clathrates has hindered optical characterization of these materials, with only one experimental report of a 0.6 eV band gap for the guest-free Ge clathrate synthesized from an ionic liquid. 20 In this work, we demonstrate the synthesis of alloyed Si-Ge type II clathrates, and discuss the behavior of the alloy system, including decomposition parameters and resultant phases.…”

Section: Introductionmentioning

confidence: 99%

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Synthesis and optical band gaps of alloyed Si–Ge type II clathrates

et al. 2014

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Inorganic type II clathrates are low density, semiconducting allotropes of group IV elements with the potential for optoelectronic applications. This class of materials is predicted to have direct or nearlydirect band gaps, and, when Si and Ge are alloyed in the clathrate structure, the band gap is tunable in the range of 0.8-1.8 eV. In this work, we demonstrate for the first time the synthesis of alloyed Si-Ge type II clathrates. Within this alloy system, we find an amorphous region which is likely due to a miscibility gap. The optical absorptance spectra of the crystalline clathrate samples show the predicted band gap tuning with Ge content, and calculations find that the Si type II clathrate has a strong absorption coefficient for the direct interband transition. The findings in this work lay the foundation for the future use of type II clathrates in optoelectronic applications.

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Section: Optical Propertiessupporting

confidence: 85%

Section: Optical Propertiesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Synthesis and optical band gaps of alloyed Si–Ge type II clathrates

et al. 2014

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“…In a recent work by Himeno et al, band gap values ranging from 1.6-1.8 eV were reported. 60 On the other hand, only one reported band gap measurement of 0.6 eV in guest-free Ge clathrate exist. The challenges associated with synthesizing guest free Ge clathrates have limited the optical data available on Ge clathrates.…”

Section: Optical Characterization Of Type II Clathratesmentioning

confidence: 99%

Group IV clathrates: synthesis, optoelectonic properties, and photovoltaic applications

et al. 2014

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Group IV clathrates are a unique class of guest/framework type compounds that are considered potential candidates for a wide range of applications (superconductors to semiconductors). To date, most of the research on group IV clathrates has focused heavily on thermoelectric applications. Recently, these materials have attracted attention as a result of their direct, wide band gaps for possible use in photovoltaic applications. Additionally, framework alloying has been shown to result in tunable band gaps. In this review, we discuss the current work and future opportunities concerning the synthesis and optical characterization of group IV clathrates for optoelectronics applications.

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“…Elemental silicon in the diamond structure (d-Si) dominates the semiconductor industry, whereas a lesser known allotrope of silicon, the Si 136 type II clathrate structure, has only recently attracted attention as a wide band gap (1.8 eV) semiconductor material. [1][2][3][4][5][6][7][8][9] Experimental demonstration of a tunable, nearly direct band gap across the Si 136−y Ge y alloy suggests the potential for photovoltaic and other optoelectronic applications. 8,9 Silicon clathrates are also actively being investigated as anode materials for lithium ion batteries due to their high charge storage capacity.…”

Section: Introductionmentioning

confidence: 99%

Efficient route to phase selective synthesis of type II silicon clathrates with low sodium occupancy

et al. 2014

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Phase selective synthesis of type II silicon clathrates from thermal decomposition of NaSi has previously been limited to small quantities due to the simultaneous formation of competing phases. In this work we show that the local sodium vapor pressure during the NaSi precursor decomposition is a critical parameter for controlling phase selection. We demonstrate synthesis techniques that allow us to tune the local Na vapor pressure, yielding type I or II clathrate products that are ≥90 wt.% phase pure. The "cold plate" reactor design discussed in this work maintains low Na vapor pressure during thermal decomposition of NaSi, thus yielding large scale, phase selective synthesis of type II silicon clathrates. The low Na vapor pressure maintained in this reactor is also shown to efficiently produce low Na (x ~1; Na x Si 136 ) Si clathrate through Na sublimation. To further reduce sodium occupancy (x < 1), we demonstrate etching of Na x Si 136 in HF/HNO 3 solutions, which rapidly yields a clathrate product with reduced x. 29 Si NMR and electron spin resonance (ESR) characterization validate the low Na occupancy of Si clathrate synthesized.The acid etch also selectively dissolves the type I silicon clathrate impurity phase, thereby enabling the synthesis of large quantities of phase pure type II silicon clathrate with low Na content.

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Optical band gap of semiconductive type II Si clathrate purified by centrifugation

Cited by 15 publications

References 15 publications

Synthesis and optical band gaps of alloyed Si–Ge type II clathrates

Synthesis and optical band gaps of alloyed Si–Ge type II clathrates

Group IV clathrates: synthesis, optoelectonic properties, and photovoltaic applications

Efficient route to phase selective synthesis of type II silicon clathrates with low sodium occupancy

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