Doped Silicon Nanocrystals Research Articles

High-efficiency electroluminescence devices containing Si nanocrystals/SiC multilayers via improved carrier injection and recombination process

Realizing high efficiency of all Si-based light-emitting devices is currently one of interesting issues in order to develop monolithic opto-electronic integration on chips. Here, we report an electroluminescence device based on phosphorus (P)-doped silicon nanocrystals (Si NCs)/silicon carbide (SiC) multilayers by modulating carrier injection and recombination process. The p+-Si substrate is used instead of p-Si substrate for facilitating the hole injection into Si NCs. Additionally, the influences of annealing temperature on the device performance have been studied, and the optimized annealing temperature is achieved by balancing the crystallinity, defect state density, and recombination process.

CORRELATION BETWEEN ELECTRICAL AND OPTICAL PROPERTIES OF DOPED SILICON NANOCRYSTALS

We propose through this work a correlation method leading to a determination of a semi-empirical relationship between optical and electrical properties in terms of refractive index and dark conductivity of doped silicon nanocrystals based on experimental data published in literature. First, an analytical model relating the conductivity and bandgap of doped silicon nanocrystals was derived. Using an empirical expression relating the refractive index to the bandgap energy, we correlated the electrical and optical parameters of N-type nanocrystalline silicon with a semi-empirical expression. The semi-empirical relationship was found to account correctly for the experimental results and yield a reasonably good agreement in an interval of the bandgap energy variation of N-type silicon nanocrystal films. The values of the fitting parameters were calculated for the N-type silicon nanocrystal films having their bandgap energy between 1. 7 eV and 2.2 eV.

Single-atom infrared emission in doped silicon nanocrystals.

Silicon luminescence, due to silicon being abundant, non-toxic and harmless, is a topic of pivotal importance in optoelectronics and biological imaging. However, a major challenge in developing high-efficiency silicon light sources is the relatively weak allowable transitions. This study focuses on single atom-doped silicon nanocrystals (Si NCs) and theoretically investigates the emission behavior of single atoms within a tetrahedral coordination field. Doping a single atom in Si NCs can result in a ∼102 times improvement at least in the squared transition dipole moment (TDM2), and induce a spectral shift towards near- and mid-infrared wavelengths. These findings offer a strong foundation for designing Si NCs for on-chip optical communication and single photon emitters.

Infrared nanoplasmonic properties of hyperdoped embedded Si nanocrystals in the few electrons regime.

Using localized surface plasmon resonance (LSPR) as an optical probe we demonstrate the presence of free carriers in phosphorus doped silicon nanocrystals (SiNCs) embedded in a silica matrix. In small SiNCs, with radius ranging from 2.6 to 5.5 nm, the infrared spectroscopy study coupled to numerical simulations allows us to determine the number of electrically active phosphorus atoms with a precision of a few atoms. We demonstrate that LSP resonances can be supported with only about 10 free electrons per nanocrystal, confirming theoretical predictions and probing the limit of the collective nature of plasmons. We reveal the appearance of an avoided crossing behavior linked to the hybridization between the localized surface plasmon in the doped nanocrystals and the silica matrix phonon modes. Finally, a careful analysis of the scattering time dependence versus carrier density in the small size regime allows us to detect the appearance of a new scattering process at high dopant concentration, which can be explained by P clustering inside the SiNCs.

Ab initio studies of the optoelectronic structure of undoped and doped silicon nanocrystals and nanowires: the role of size, passivation, symmetry and phase.

Silicon nanocrystals and nanowires have been extensively studied because of their novel properties and their applications in electronic, optoelectronic, photovoltaic, thermoelectric and biological devices. Here we discuss results from ab initio calculations for undoped and doped Si nanocrystals and nanowires, showing how theory can aid and improve comprehension of the structural, electronic and optical properties of these systems.

Probing Dopant Locations in Silicon Nanocrystals via High Energy X-ray Diffraction and Reverse Monte Carlo Simulation.

Understanding the locations of dopant atoms in ensembles of nanocrystals is crucial to controlling the dopants' function. While electron microscopy and atom probe tomography methods allow investigation of dopant location for small numbers of nanocrystals, assessing large ensembles has remained a challenge. Here, we are using high energy X-ray diffraction (HE-XRD) and structure reconstruction via reverse Monte Carlo simulation to characterize nanocrystal structure and dopant locations in ensembles of highly boron and phosphorus doped silicon nanocrystals (Si NCs). These plasma-synthesized NCs are a particularly intriguing test system for such an investigation, as elemental analysis suggests that Si NCs can be "hyperdoped" beyond the thermodynamic solubility limit in bulk silicon. Yet, free carrier densities derived from local surface plasmon resonances suggest that only a fraction of dopants are active. We demonstrate that the structural characteristics garnered from HE-XRD and structure reconstruction elucidate dopant location and doping efficacy for doped Si NCs from an atomic-scale perspective.

Synthesis of PEG-grafted boron doped Si nanocrystals.

Silicon nanocrystals are intriguing materials for biomedical imaging applications because of their unique optical properties and biological compatibility. We report a new surface functionalization route to synthesize biological buffer soluble and colloidally stable silicon nanocrystals, which is enabled by surface boron doping. Harnessing the distinctive Lewis acidic boron surface sites, postsynthetic modifications of plasma synthesized boron doped nanocrystals were carried out with polyethylene glycol (PEG-OH) ligands in dimethyl sulfoxide under photochemical conditions. The influence of PEG concentration, PEG molecular weight, and boron doping percentage on the nanocrystal solubility in a biological buffer has been investigated. The boron doping facilitates the surface functionalization via two probable pathways, by providing excellent initial dispersiblity in polar solvents and providing available acidic boron surface sites for bonding. These boron doped silicon nanocrystals have nearly identical absorption features as intrinsic silicon nanocrystals, indicating that they are promising candidates for biological imaging applications.

Synaptic silicon-nanocrystal phototransistors for neuromorphic computing

The incorporation of augmentative functionalities into a single synaptic device is greatly desired to enhance the performance of neuromorphic computing, which has brain-like high intelligence and low energy consumption. This encourages the development of multi-functional synaptic devices with architectures that are capable of achieving demanded synaptic plasticity. Here we take advantage of the remarkable optical absorption of boron (B)-doped silicon nanocrystals (Si NCs) to make synaptic phototransistors, which can be stimulated by both optical and electrical spikes. The optical and electrical stimulations enable a series of important synaptic functionalities for the synaptic Si-NC phototransistors, well mimicking biological synapses. It is interesting that the synergy of the photogating and electrical gating of the synaptic Si-NC phototransistors leads to the implementation of aversion learning and logic functions. We show that a spiking neural network based on the synaptic Si-NC phototransistors may be trained for the recognition of handwritten digits in the modified national institute of standards and technology (MNIST) database with a recognition accuracy around 94%. The energy consumption of the synaptic Si-NC phototransistors may be rather low, which should help advance energy-efficient neuromorphic computing.

Critical Size for Carrier Delocalization in Doped Silicon Nanocrystals: A Study by Ultrafast Spectroscopy

We present a comprehensive ultrafast spectroscopy-based study on the delocalization of doping-induced carriers in Si nanocrystals (NCs). To this end we prepare thin films of differently sized doped Si NCs and vary the doping configurations from singly P and B doping to simultaneously P and B co-doping. We show that the NC size orchestrates the level of delocalization of the doping-induced carriers. This can be understood in light of (1) the quantum confinement effect and (2) unscreened Coulomb interactions by moving further into the nanoscale. Both contributions affect the activation energy (ΔE) that is required to create free majority carriers. By varying the NC size in combination with the doping configuration we tune ΔE and control the delocalization of the doping-induced carriers. Most importantly, we show that there is a critical NC diameter of Dcritical ≈ 6 nm that describes the transition from a localized to a free carrier regime. In particular, our results show that optical bandgaps of ∼0.95 eV (optimal for carrier multiplication-facilitated solar cell power conversion) can be achieved in P–B co-doped Si NCs with DNC < Dcritical. These results indicate that the practical photovoltaic feasibility of co-doped Si NCs is not limited by the presence of some remaining free carriers in uncompensated NCs.

Toxicity Evaluation of Boron- and Phosphorus-Doped Silicon Nanocrystals toward Shewanella oneidensis MR-1

Silicon nanocrystals, also known as silicon quantum dots, are regarded as green alternatives to traditional quantum dots composed of heavy metal elements. While it is well-known that the semiconductor properties of these materials can be tuned by doping with p/n-type dopants (i.e., boron and phosphorus), there is a lack of systematic understanding of their potential environmental impact if released into the ecosystem. Here, we demonstrate that introduction of dopants, especially phosphorus, cause doped silicon nanocrystals to produce reactive oxygen species, resulting in significant toxicity to a model microorganism, Shewanella oneidensis MR-1. In addition, the interaction between bacteria cells and silicon nanocrystals was investigated using dark field microscopy and bio-TEM. Interestingly, boron-doped silicon nanocrystals tended to attach to the cell surface while this phenomenon was not observed for undoped or phosphorus-doped silicon nanocrystals.

Type-I alignment in MAPbI3 based solar devices with doped-silicon nanocrystals

In this work we couple silicon nanocrystals (Si NCs) with strong quantum confinement (< 4 nm diameter) with large grained MAPbI3 films combining the perovskites excellent transport properties with the unique opto-electronic properties of quantum confined Si NCs. The electronic structures of MAPbI3 and Si NCs, ideally aligned to form a type-I heterojunction, demonstrated improved carrier collection through a higher short-circuit current density (~ 20 mA cm−2) measured in photovoltaic test devices. The addition of p- and n-doped Si NCs within the MAPbI3 films also showed unexpected and interesting device performance improvements, where the typical perovskite degradation process in ambient conditions was considerably slowed down and the overall device efficiency was observed to increase under light soaking. We attribute these improvements to the presence of Si NCs, which helped maintain the perovskite film qualities for more than 14 days in open atmosphere.

Nonlinear Doping, Chemical Passivation and Photoluminescence Mechanism in Water-Soluble Silicon Quantum Dots by Mechanochemical Synthesis**Supported by the National Natural Science Foundation of China under Grant No 61575216.

A series of boron- and phosphorus-doped silicon wafers are used to prepare a series of doped silicon nanocrystals (nc-Si) by high-energy ball milling with carboxylic acid-terminated surface. The sizes of the nc-Si samples are demonstrated to be <5 nm. The doping levels of the nc-Si are found to be nonlinearly dependent on the original doping level of the wafers by x-ray photoelectron spectroscopy measurement. It is found that the nonlinear doping process will lead to the nonlinear chemical passivation and photoluminescence (PL) intensity evolution. The doping, chemical passivation and PL mechanisms of the doped nc-Si samples prepared by mechanochemical synthesis are analyzed in detail.

Synthesis, optical properties and theoretical modelling of discrete emitting states in doped silicon nanocrystals for bioimaging.

The creation of multiple emission pathways in quantum dots (QDs) is an exciting prospect with fundamental interest and optoelectronic potential. For the first time, we report multiple emission pathways in semiconductor nanocrystals (NCs) where the number of emission pathways desired is controlled by the number of dopant atoms per quantum dot. The origin of additional emission pathways is explained by interactions between dopant states and NC energy levels. Density functional theory (DFT) calculations of undoped 2.3 nm silicon (Si NCs) and the same NCs doped with 2 interstitial Cu atoms show good agreement to experiment. Such calculations provide valuable data to explain the changes in optical transitions due to the Cu dopant in terms of transition energies, quantum yield and dopant position as a function of dopants per NC. Changes in the optical properties of Si NCs induced by dopant concentration include extended excitation range and enhanced absorption coefficients, emission redshifts of up to 60 nm, and a two-fold increase in quantum yields up to 22%. The optical properties of doped NCs lead to significant bioimaging improvements illustrated by in vitro cell imaging, including redshifted excitation wavelengths away from natural autofluorescence and enhanced fluorescent signals.

Free electron-driven photophysics in n-type doped silicon nanocrystals.

Although there have been extensive speculation regarding the applicability of doped silicon nanocrystals (Si NCs) in optoelectronic technologies, a quantitative analysis on the photophysical workings of introduced free carriers remains elusive. Here, we present a comprehensive study on the photophysics of ∼7.5 nm heavily phosphorous-doped Si NCs, using a combination of spectroscopic techniques. We correlate the carrier dynamics with the location of the free carriers - which we tune from NC core to surface depending on the state of oxidation. The strength of the Coulomb interactions between the photoexcited electron-hole pairs and the doping-induced free carriers depends on (1) the concentration of free carriers, (2) the location of these carriers, and (3) the diameter of the NCs. In contrast to prior studies, the photoexcited carrier dynamics in these n-type doped Si NCs are dominated by strong Coulomb interactions with doping-induced free electrons, characterized by a negative trion lifetime of around 9 ns. While radiative recombination in doped direct bandgap NCs can often still compete with trion recombination (allowing emission to be present), emission in our doped Si NCs is completely quenched due to the relatively slow radiative recombination in these indirect bandgap NCs. Furthermore, multi-exciton interaction times are slightly shortened compared to those of intrinsic Si NCs, which we attribute to an increased number of free electrons, enhancing the oscillator strength of Auger recombination. These results constitute a framework for the optimization of doped Si NC synthesis techniques and device engineering directions for future doped-Si NC-based optoelectronic and photovoltaic applications.

Tailoring CO2 Reduction with Doped Silicon Nanocrystals

AbstractMore than 20 gigatonnes of carbon dioxide are released into the atmosphere every year. The conversion of CO2 into value‐added chemicals and fuels by solar energy is an immediate solution to mitigate CO2 emissions, while providing global energy security. In this work, boron‐ and phosphorus‐doped silicon nanocrystals (ncSi), comprised of three earth‐abundant elements, are investigated for gas‐phase heterogeneous photoreduction of CO2 for the first time. Surface dopants are demonstrated to induce CO2 adsorption capacity. Remarkably, phosphorus‐doped ncSi is found to be the best performer among the singly doped and co‐doped ncSi samples, doubling the rate of pristine ncSi. The enhancement of activity is attributed to the combination of the number of surface hydrides, its surface hydrophobicity, the addition of electronegative surface atoms, and perhaps an enhanced hydridic character of the SiH induced by the n‐doping effect. Significantly, boron and phosphorus dopants are shown to provide increased stability of CO2 reduction activity compared to pristine ncSi after storing the samples in air for 2 weeks. These noteworthy findings open up a pathway to develop sustainable alternatives for existing photocatalysts for CO2 conversion.

Microstructure and photoelectric properties of P-doped silicon-rich SiNx film as an n-type layer for PIN-type amorphous silicon thin film solar cells

In this study, the phosphorus doped silicon nanocrystals embedded in silicon-rich SiNx:H thin films were fabricated using a radio frequency plasma enhanced chemical vapor deposition technique, from a SiH4, pH3, H2 and NH3 mixture. The effects of nitrogen incorporation on the microstructure and photoelectric properties of the nc-Si:H film were systematically investigated. Raman and transmission electron microscope observation revealed the fact that nitrogen incorporation inhibited the growth of Si nanocrystals, and their sizes and densities of them could be adjusted by varying the incorporation flow ratio NH3/(SiH4+H2+pH3). The reduction of refractive index and improvement of conductivity of the prepared films were observed with increasing nitrogen impurity, and a maximal conductivity was obtained when the flow ratio was 0.02. In addition, the P-doped nc-SiNx:H film was used as an n-type layer for the amorphous silicon thin film solar cells to optimize its efficiency, and the solar cell with high fill factor (0.624) and efficiency (9.26%) has been obtained when the flow ratio was 0.02. As a result, the P-doped nc-SiNx:H film with proper nitrogen incorporation was able to efficiently improve the performance of the thin film solar cells.

Ion Beam Synthesis of Doped Nanocrystals of Si1-xGex Alloys Embedded in SiO2

ABSTRACTAs an extension of our previous proving that ion beam synthesis is an efficient route to form doped silicon nanocrystals (nc’s) [1, 2], we show here that ion beam synthesis, by co-implantion of the dopant and of the constituents of the alloy, followed by a single high temperature anneal, is also a convenient way to grow more complex structures, such as As doped nc’s of Si1-xGex alloys.Rutherford backscattering spectrometry (RBS) is used to measure the impurity profiles and evaluate the average concentration of the various species (Si, Ge, As). The formation of the nc’s is evidenced by TEM observation and further confirmed by Raman and XRD analysis, which also allow us to estimate the Ge content of the nc’s.The incorporation of As inside the Si1-xGex nc’s is attested by the presence of the characteristic Ge-As related line in the Raman spectra of the As-doped samples, and strengthened by the increased conductivity of MOS structure including such doped nc’s inside the dielectric film.

Atom probe tomography of size‐controlled phosphorus doped silicon nanocrystals

Doping of silicon nanocrystals is essential to control their electronic and optical properties. The incorporation of an impurity into a silicon nanovolume is a nontrivial task due to the self‐purification effect. Here, a systematic atom probe tomography study of the phosphorus distribution and incorporation in size‐controlled silicon nanocrystals embedded in silicon dioxide is presented. Qualitatively, it turns out that the phosphorus distribution in the system follows a universal, nanocrystal‐size independent trend: phosphorus‐enrichment at the interface with a substantial phosphorus‐incorporation in the silicon nanocrystal as small as 2 nm in diameter. This clearly contradicts strict self‐purification. These observations are explained by the bulk‐solubility and ‐segregation behaviour, kinetic effects related to the diffusion lengths, and nanoscale interface strain. The quantitative determination of the amount of phosphorus atoms per quantum dot enables a systematic understanding of phosphorus‐induced effects on optical and electronic properties of silicon nanovolumes.

Structural, optical, and electrical properties of silicon nanocrystals fabricated by high silicon content silicon-rich oxide and silicon dioxide bilayers

Intrinsic, boron (B)-doped, and phosphorus (P)-doped silicon nanocrystals (Si NCs) formed from an excess Si concentration of 40 at. % were investigated to study their structural, optical, and electrical properties. Atom probe tomography (APT) revealed that the size and arrangement of Si NCs were different in each sample. A strong blue shift in photoluminescence spectra for the intrinsic and B-doped Si NCs was correlated with the volume fraction of small Si NCs. The lower resistivity of the B-doped sample than the P-doped one was explained by the percolation of Si NCs through the film.

Ligand-Free, Colloidal, and Plasmonic Silicon Nanocrystals Heavily Doped with Boron

Colloidal heavily doped silicon nanocrystals (Si NCs) exhibiting tunable localized surface plasmon resonance (LSPR) are of great interest in cost-effective, solution-processed optoelectronic devices given the abundance and nontoxicity of Si. In this work we show that tunable plasmonic properties and colloidal stability without the use of ligands can be simultaneously obtained for Si NCs heavily doped with boron (B). The heavily B-doped Si NC colloids are found to be stable in air for months, opening up the possibility of device processing in ambient atmosphere. The optical absorption of heavily B-doped Si NCs reveals that the heavy B doping not only changes the concentration of free carriers that are confined in Si NCs but also modifies the band structure of Si NCs. After heavy B doping both indirect and direct electronic transition energies remarkably decrease in Si NCs because the heavy B doping induced movement of the conduction band toward the band gap could be more significant than that of the Fermi ...