Zhenyi Ni

Comparative Study on the Localized Surface Plasmon Resonance of Boron- and Phosphorus-Doped Silicon Nanocrystals

Localized surface plasmon resonance (LSPR) of doped Si nanocrystals (NCs) is critical to the development of Si-based plasmonics. We now experimentally show that LSPR can be obtained from both B- and P-doped Si NCs in the mid-infrared region. Both experiments and calculations demonstrate that the Drude model can be used to describe the LSPR of Si NCs if the dielectric screening and carrier effective mass of Si NCs are considered. When the doping levels of B and P are similar, the LSPR energy of B-doped Si NCs is higher than that of P-doped Si NCs because B is more efficiently activated to produce free carriers than P in Si NCs. We find that the plasmonic coupling between Si NCs is effectively blocked by oxide at the NC surface. The LSPR quality factors of B- and P-doped Si NCs approach those of traditional noble metal NCs. We demonstrate that LSPR is an effective means to gain physical insights on the electronic properties of doped Si NCs. The current work on the model semiconductor NCs, i.e., Si NCs has important implication for the physical understanding and practical use of semiconductor NC plasmonics.

Silicene oxides: formation, structures and electronic properties

Understanding the oxidation of silicon has been critical to the success of all types of silicon materials, which are the cornerstones of modern silicon technologies. For the recent experimentally obtained two-dimensional silicene, oxidation should also be addressed to enable the development of silicene-based devices. Here we focus on silicene oxides (SOs) that result from the partial or full oxidation of silicene in the framework of density functional theory. It is found that the formation of SOs greatly depends on oxidation conditions, which concern the oxidizing agents of oxygen and hydroxyl. The honeycomb lattice of silicene may be preserved, distorted or destroyed after oxidation. The charge state of Si in partially oxidized silicene ranges from +1 to +3, while that in fully oxidized silicene is +4. Metals, semimetals, semiconductors and insulators can all be found among the SOs, which show a wide spectrum of electronic structures. Our work indicates that the oxidation of silicene should be exquisitely controlled to obtain specific SOs with desired electronic properties.

Plasmonic Silicon Quantum Dots Enabled High-Sensitivity Ultrabroadband Photodetection of Graphene-Based Hybrid Phototransistors

Highly sensitive photodetection even approaching the single-photon level is critical to many important applications. Graphene-based hybrid phototransistors are particularly promising for high-sensitivity photodetection because they have high photoconductive gain due to the high mobility of graphene. Given their remarkable optoelectronic properties and solution-based processing, colloidal quantum dots (QDs) have been preferentially used to fabricate graphene-based hybrid phototransistors. However, the resulting QD/graphene hybrid phototransistors face the challenge of extending the photodetection into the technologically important mid-infrared (MIR) region. Here, we demonstrate the highly sensitive MIR photodetection of QD/graphene hybrid phototransistors by using plasmonic silicon (Si) QDs doped with boron (B). The localized surface plasmon resonance (LSPR) of B-doped Si QDs enhances the MIR absorption of graphene. The electron-transition-based optical absorption of B-doped Si QDs in the ultraviolet (UV) to near-infrared (NIR) region additionally leads to photogating for graphene. The resulting UV-to-MIR ultrabroadband photodetection of our QD/graphene hybrid phototransistors features ultrahigh responsivity (up to ∼109 A/W), gain (up to ∼1012), and specific detectivity (up to ∼1013 Jones).

Freestanding doped silicon nanocrystals synthesized by plasma

Freestanding silicon nanocrystals (Si NCs) have recently gained great popularity largely due to their easily accessible surface and flexible incorporation into device structures. In the past decade plasmas have been increasingly employed to synthesize freestanding Si NCs. As freestanding Si NCs move closer to applications in a variety of fields such as electronics, thermoelectrics and lithium-ion batteries, doping becomes more imperative. Such a context explains the current great interest in plasma-synthesized doped freestanding Si NCs. In this work we review the synthesis of freestanding doped Si NCs by plasma. Doping-induced structural, electronic, optical and oxidation properties of Si NCs are discussed. We also review the applications of plasma-synthesized doped freestanding Si NCs that have been demonstrated so far. The development of freestanding doped Si NCs synthesized by plasma in the future is envisioned.

Density functional theory study on organically surface-modified silicene

Organic surface modification may be critical to the practical use of silicene, which is a novel two-dimensional layered material. It is intriguing to know if organic surface modification seriously impacts the structural, electronic and optical properties of silicene. In this work, we focus on four hydrogenation-based organic surface modification schemes (hydrosilylation, alkoxylation, aminization and phenylation) with the experimentally demonstrated surface coverage of ∼33%. The geometrical structures, band structures and optical absorption of organically surface-modified silicene have been compared with those of silicene and hydrogenated silicene (H-silicene) in the framework of density functional theory. It is found that organic surface modification leads to the increase of the buckling distance of silicene, while causing the angles of bonds in the honeycomb structure of silicene to decrease. Although the initial hydrogenation makes silicene become an indirect-bandgap semiconductor, the subsequent organic surface modification schemes further change the band structure of silicene. Hydrosilylation, phenylation, alkoxylation and amination all give rise to the reduction of the bandgap of H-silicene. Hydrosilylated and phenylated silicene are indirect-bandgap semiconductors, while alkoxylated and aminated silicene are direct-bandgap semiconductors. Changes of the optical absorption induced by organic surface modification are well correlated to the corresponding changes of the band structure.

Low-resistivity bulk silicon prepared by hot-pressing boron- and phosphorus-hyperdoped silicon nanocrystals

Technologically important low-resistivity bulk Si has been usually produced by the traditional Czochralski growth method. We now explore a novel method to obtain low-resistivity bulk Si by hot-pressing B- and P-hyperdoped Si nanocrystals (NCs). In this work bulk Si with the resistivity as low as ∼ 0.8 (40) mΩ•cm has been produced by hot pressing P (B)-hyperdoped Si NCs. The dopant type is found to make a difference for the sintering of Si NCs during the hot pressing. Bulk Si hot-pressed from P-hyperdoped Si NCs is more compact than that hot-pressed from B-hyperdoped Si NCs when the hot-pressing temperature is the same. This leads to the fact that P is more effectively activated to produce free carriers than B in the hot-pressed bulk Si. Compared with the dopant concentration, the hot-pressing temperature more significantly affects the structural and electrical properties of hot-pressed bulk Si. With the increase of the hot-pressing temperature the density of hot-pressed bulk Si increases. The highest carr...

Light-Emitting Diodes Based on Colloidal Silicon Quantum Dots with Octyl and Phenylpropyl Ligands

Colloidal silicon quantum dots (Si QDs) hold ever-growing promise for the development of novel optoelectronic devices such as light-emitting diodes (LEDs). Although it has been proposed that ligands at the surface of colloidal Si QDs may significantly impact the performance of LEDs based on colloidal Si QDs, little systematic work has been carried out to compare the performance of LEDs that are fabricated using colloidal Si QDs with different ligands. Here, colloidal Si QDs with rather short octyl ligands (Octyl-Si QDs) and phenylpropyl ligands (PhPr-Si QDs) are employed for the fabrication of LEDs. It is found that the optical power density of PhPr-Si QD LEDs is larger than that of Octyl-Si QD LEDs. This is due to the fact that the surface of PhPr-Si QDs is more oxidized and less defective than that of Octyl-Si QDs. Moreover, the benzene rings of phenylpropyl ligands significantly enhance the electron transport of QD LEDs. It is interesting that the external quantum efficiency (EQE) of PhPr-Si QD LEDs is lower than that of Octyl-Si QD LEDs because the benzene rings of phenylpropyl ligands suppress the hole transport of QD LEDs. The unbalance between the electron and hole injection in PhPr-Si QD LEDs is more serious than that in Octyl-Si QD LEDs. The currently obtained highest optical power density of ∼0.64 mW/cm2 from PhPr-Si QD LEDs and highest EQE of ∼6.2% from Octyl-Si QD LEDs should encourage efforts to further advance the development of high-performance optoelectronic devices based on colloidal Si QDs.

Density functional theory study on a 1.4 nm silicon nanocrystal coated with carbon

Charge carrier transport associated with silicon nanocrystals (Si NCs) can be improved by removing hydrocarbon chains that are routinely attached to the NC surface by means of hydrosilylation. Thermal annealing for the hydrocarbon-chain removal may lead to carbon-coated Si NCs. But the optical behavior of carbon-coated Si NCs has not been clearly understood. By comparing a carbon-coated Si NC with those fully passivated by hydrogen (H) or coated with silicon oxide (SiO2) in the framework of density functional theory, we find that carbon coating causes both the excitation energy and emission energy of the Si NC to significantly decrease. The carbon-coated Si NC exhibits a smaller Stokes shift than the fully H-passivated and SiO2-coated Si NCs. The radiative recombination rate of the carbon-coated Si NC is two orders of magnitude lower than those of the fully H-passivated and SiO2-coated Si NCs. The thermal removal of hydrocarbon chains at the NC surface is not recommended for Si-NC-based light-emitting devices because carbon-coated Si NCs with rather low light emission efficiency may be produced. In contrast, the carbon coating of Si NCs may be beneficial for Si-NC-based solar cells.

Twinned silicon and germanium nanocrystals: Formation, stability and quantum confinement

Although twins are often observed in Si/Ge nanocrystals (NCs), little theoretical investigation has been carried out to understand this type of important planar defects in Si/Ge NCs. We now study the twinning of Si/Ge NCs in the frame work of density functional theory by representatively considering single-twinned and fivefold-twinned Si/Ge NCs. It is found that the formation of twinned Si/Ge NCs is thermodynamically possible. The effect of twinning on the formation of Si NCs is different from that of Ge NCs. For both Si and Ge NCs twinning enhances their stability. The quantum confinement effect is weakened by twinning for Si NCs. Twinning actually enhances the quantum confinement of Ge NCs when they are small ( 136 atoms). The current results help to better understand the experimental work on twinned Si/Ge NCs and guide the tuning of Si/Ge-NC structures for desired properties.

Density functional theory study on the boron and phosphorus doping of germanium quantum dots

Doping is a crucial way of tuning the properties of semiconductor quantum dots (QDs). As one type of important semiconductor QDs, germanium (Ge) QDs have been recently doped with boron (B) and phosphorus (P) through a gas-phase synthesis approach successfully. However, theoretical understanding about the structural and electronic properties of doped Ge QDs remains rather limited. Here we investigate the doping of Ge QDs with B and P in the framework of density functional theory. The formation energies and electronic structures of singularly B- or P-doped Ge QDs and B/P-codoped Ge QDs are systematically studied. It is found that both B and P prefer the near-surface region of Ge QDs. The electronic structures of Ge QDs can be effectively tuned by B and P doping. The B/P codoping may facilitate the incorporation of B and P into Ge QDs, resulting in the further modification of the electronic structures of Ge QDs.