Xiaodong Pi

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.

Graphene Coupled with Silicon Quantum Dots for High-Performance Bulk-Silicon-Based Schottky-Junction Photodetectors.

Graphene is coupled with silicon quantum dots (Si QDs) on top of bulk Si to form a hybrid photodetector. Si QDs cause an increase of the built-in potential of the graphene/Si Schottky junction while reducing the optical reflection of the photodetector. Both the electrical and optical contributions of Si QDs enable a superior performance of the photodetector.

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).

Efficient silicon quantum dots light emitting diodes with an inverted device structure

We use silicon quantum dots (SiQDs) with an average diameter of 2.6 ± 0.5 nm as the light emitting material and fabricate inverted structure light emitting diodes (SiQD-LEDs) with bottom cathodes. ZnO nanoparticles with high electron mobility, a deep valence band edge, and robust features to resist dissolving by the SiQD solvent were used as the electron transport layer. 1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) with high hole transport mobility and a high lowest unoccupied molecular orbital level was used as the hole transport layer. Poly(ethylene imine) (PEI) modified indium-tin oxide (ITO) was used as the low work function (∼3.1 eV) cathode and MoO3/Al as the high work function anode. Electroluminescence of the SiQD-LEDs is mainly from the SiQDs with a peak located at ∼700 nm. The maximum external quantum efficiencies of the SiQD-LEDs are 2.7%.

A self-powered high-performance graphene/silicon ultraviolet photodetector with ultra-shallow junction: breaking the limit of silicon?

This work is supported by National Science Foundation (DMR1508144), NSFC (Grant Nos. 61274123, 61474099, 61674127,and 61431014), and micro-fabrication/nano-fabrication platform of ZJU University, and the Fundamental Research Funds for the Central Universities (2016XZZX001-05). This work is also supported by ZJU Cyber Scholarship and Cyrus Tang Center for Sensor Materials and Applications, the Open Research Fund of State Key Laboratory of Bioelectronics, Southeast University, the Open Research Fund of State Key Laboratory of Nanodevices and Applications at Chinese Academy of Sciences (No.14ZS01), and Visiting-by-Fellowship of Churchill College at University of Cambridge.

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.

Doping silicon nanocrystals with boron and phosphorus

The properties of silicon nanocrystals (Si NCs) that are usually a few nanometers in size can be exquisitely tuned by boron (B) and phosphorus (P) doping. Recent progress in the simulation of B- and P-doped Si NCs has led to improved explanation for Band P-doping-induced changes in the optical properties of Si NCs. This is mainly enabled by comprehensive investigation on the locations of B and P in Si NCs and the electronic properties of B- and P-doped Si NCs. I remarks on the implications of newly gained insights on B- and P-doped Si NCs. Continuous research to advance the understanding of the doping of Si NCs with B and P is envisioned.

A Broadband Fluorographene Photodetector

High-performance photodetectors operating over a broad wavelength range from ultraviolet, visible, to infrared are of scientific and technological importance for a wide range of applications. Here, a photodetector based on van der Waals heterostructures of graphene and its fluorine-functionalized derivative is presented. It consistently shows broadband photoresponse from the ultraviolet (255 nm) to the mid-infrared (4.3 µm) wavelengths, with three orders of magnitude enhanced responsivity compared to pristine graphene photodetectors. The broadband photodetection is attributed to the synergistic effects of the spatial nonuniform collective quantum confinement of sp2 domains, and the trapping of photoexcited charge carriers in the localized states in sp3 domains. Tunable photoresponse is achieved by controlling the nature of sp3 sites and the size and fraction of sp3 /sp2 domains. In addition, the photoresponse due to the different photoexcited-charge-carrier trapping times in sp2 and sp3 nanodomains is determined. The proposed scheme paves the way toward implementing high-performance broadband graphene-based photodetectors.

Enhancement of electroluminescence from TiO2/p+-Si heterostructure-based devices through engineering of oxygen vacancies in TiO2

We report that electroluminescence (EL) from TiO2/p+-Si heterostructure-based devices can be significantly enhanced through a prior treatment of TiO2 films in argon (Ar) plasma. It is found that the Ar-plasma treatment introduces excess oxygen vacancies within a certain depth of TiO2 films. The increase in the concentration of oxygen vacancies leads to the enhancement of EL from TiO2/p+-Si heterostructure-based devices because oxygen vacancies are the light-emitting centers. This work demonstrates the use of defect engineering to improve the performance of oxide-based optoelectronic devices.