Colloidal Silicon Quantum Dots Research Articles

(Invited) Synthesis of Ultra-Bright Colloidal Silicon Quantum Dots (PLQY~80%) and Highly Efficient LEDs (EQE>10%)

Silicon is the second most abundant element in the Earth’s crust and could be considered an environmentally benign material. Silicon has been widely used in electronic devices and solar cells, and it has dominated the semiconductor research and development. However, the photoluminescence (PL) of the bulk material shows an invisible NIR wavelength (λ ~ 1100 nm). In addition, the PL quantum yield (PLQY) of bulk crystal Si is ~0.01% owing to the indirect transition nature. In contrast, silicon quantum dots show PL in the entire visible spectral region [1]. Furthermore, colloidal silicon quantum dots demonstrate PLQYs of 60% [2], and thereby they give highly impact as environmentally benign and heavy-metal-free quantum dots (i.e., In, Cd, and Pb free) for displays, lighting, and biomedical imaging via solution processing. Indeed, silicon quantum dot LEDs have recently been produced from a natural substance, by upcycling discarded rice husks [3].Herein, red PL with a PLQY of 60–80% and LEDs with an external quantum efficiency of >10% were obtained at 1/3600th of the cost of existing methods. This was achieved by using trichlorosilane and an organic solvent to prepare a hydrogen silsesquioxane (HSQ) polymer, and by optimizing the LED optoelectric properties. Based on many silicon quantum dots synthesized in our study, each of them enabled us to quantitative the relationships HSQ polymer structure, PLQY, and LED efficiency, exhibiting the critical parameters that governed their performances. Moreover, the simple and cost-effective methods proposed here could prove invaluable for the synthesis and characterization of silicon quantum dots in the near future.

Not all silicon quantum dots are equal: photostability of silicon quantum dots with and without a thick amorphous shell.

Luminescent colloidal silicon quantum dots (SiQDs) are sustainable alternatives to metal-based QDs for various optical applications. While the materials are reliant on their photoluminescence efficiency, the relationship between the structure and photostability of SiQDs is yet to be well studied. An amorphous silicon (a-Si) shell was recently discovered in SiQDs prepared by thermally-processed silicon oxides. As a-Si is known as a source of defects upon UV irradiation, the disordered shell could potentially have an adverse effect on the optical properties of nanoparticles. Herein, the photostability of ∼5 nm diameter SiQDs with an amorphous shell was compared with that of over-etched SiQDs of equivalent dimensions that bore an a-Si shell of negligible thickness. An UV-induced degradation study was conducted by subjecting toluene solutions of SiQDs to 365 nm light-emitting diodes (LEDs) under an inert atmosphere for predetermined times up to 72 hours. The structure, composition, and optical responses of the exposed SiQDs were evaluated.

Cost-Effective Ultrabright Silicon Quantum Dots and Highly Efficient LEDs from Low-Carbon Hydrogen Silsesquioxane Polymers.

Cost-effective methods of synthesizing bright colloidal silicon quantum dots (SiQDs) for use as heavy-metal-free QDs, which have applications as light sources in biomedicine and displays, are required. We report simple protocols for synthesizing ultrabright colloidal SiQDs and fabricating SiQD LEDs based on hydrogen silsesquioxane (HSQ) polymer synthesis. Red photoluminescence with a quantum yield (PLQY) of 60-80% and LEDs with an external quantum efficiency (EQE) of >10% were obtained at 1/3600th of the cost of existing methods. This was achieved by using HSiCl3 and a low-polarity solvent to prepare the HSQ polymer and by optimizing the LED hole-injection layer thickness. A stochastic analysis of 31 SiQD syntheses revealed that SiQDs with the highest PLQYs were obtained from a hard, low-carbon HSQ polymer precursor containing many Si-H groups and cage structures. Notably, simple FTIR measurements predicted whether a HSQ polymer would yield high-PLQY SiQDs and high-EQE LEDs. These straightforward, cost-effective protocols should lead to advances in SiQD synthesis and LED fabrication methods.

Colloidal Silicon Quantum Dot‐Based Cavity Light‐Emitting Diodes with Narrowed and Tunable Electroluminescence

AbstractLuminescent colloidal silicon quantum dots (SiQDs) are explored as alternatives to metal‐based quantum dots for light‐emitting diodes (LEDs) because of the abundance and biocompatibility of silicon. To date, the broad electroluminescence (EL) bandwidth (>100 nm) and blueshifting of EL at high applied voltages of SiQD‐LEDs are outstanding challenges that limit a competitive spectral purity and device stability. Herein, the fabrication and testing of SiQD‐LEDs that incorporate a Fabry–Pérot cavity are reported which exhibit a narrow spectral linewidth as low as ≈23 nm. The presented devices also provide wavelength and visual stability from +4 to +8 V, as well as spectral tunability.

Luminescence-guided and visibly transparent solar concentrators based on silicon quantum dots.

In this work, we demonstrate a new tapered prism-shaped luminescent solar concentrator (LSC), which guides most of the luminescence toward one edge instead of four, for the solar window application. Only one Si photovoltaic (PV) strip attached to the light-emitting sidewall is needed to collect the luminescence, which further reduces PV material cost and avoids electrical mismatch. To achieve high visible transmission and mitigate reabsorption, colloidal silicon quantum dots (SiQDs) with ultraviolet-selective absorption and large Stokes shift are used as the fluorophores. With the SiQD concentration equal to 8 mg mL-1, the SiQD-LSC as a solar window can attain a power conversion efficiency (PCE) equal to 0.27%, while ensuring high average visible transmission (AVT = 86%) and high color rendering index (CRI = 94 with AM1.5G as the incident spectrum). When adjusted to front-facing, the Si PV strip can harvest not only the direct sunlight but also the concentrated SiQD fluorescence guided from the LSC. As a result, the overall solar window PCE can be increased to 1.18%, and the PCE of the front-facing Si PV strip alone can be increased by 7% due to the luminescence guided from the SiQD-LSC.

(Invited) Realizing Narrow Bandwidth Visible Photoluminescence from Colloidal Silicon Quantum Dots

Quantum dot light-emitting diodes (QD-LEDs) are an attractive alternative to organic light-emitting diodes (OLEDs) that have attracted attention for next generation optoelectronic devices due to their high colour-saturated photoluminescence, processability, and stability. Existing QD-LEDs often employ toxic or rare metals (e.g., Cd, Pb, In, etc.). Although colloidal silicon quantum dots (SiQDs) are an attractive alternative because of their abundance, biocompatibility and tailorable surface chemistry, tuning their luminescence throughout the visible spectrum can be challenging. Furthermore, SiQD emission shows a comparatively wide bandwidth that is a consequence of the inherent properties of the Si band structure. As a result the narrowing SiQD luminescence is not readily achieved by straightforward size selection methods used for other quantum dots. One alternative approach is to prepare optical cavities that preferentially select specific emission wavelengths using optical structures. This presentation will outline our investigations targeted on realizing bright, high quantum yield, narrow bandwidth luminescence from SiQD-polymer hybrids.

Ligand Effects on Photoluminescence and Electroluminescence of Silicon Quantum Dots for Light-Emitting Diodes

Colloidal silicon quantum dots (SiQDs) may potentially minimize the environmental impact of commercial LEDs and advance next-generation light sources. Many studies have investigated the optical properties of SiQDs prepared by chemical synthesis, but the essential features of surface ligands have not fully been understood. Characterizing surface ligands should have a significant impact on optoelectronic research and ensuing applications. In this study, colloidal SiQDs were synthesized by pyrolyzing hydrogen silsesquioxane, followed by thermal hydrosilylation with 1-decene. Decyl-terminated SiQDs exhibited photoluminescence (PL) in a wavelength of 730 nm and PL quantum yields (QYs) of up to 38%. Seven decyl-terminated SiQDs with different ligand coverages were synthesized by varying the reaction time of hydrosilylation between 10 min and 9 h, and then these SiQDs were assembled into LEDs. The PL spectra, PLQYs, and performance of the SiQD LEDs were evaluated as a function of the decyl-ligand coverage. The PL properties (i.e., peak wavelength and PLQY) were insensitive to changes in decyl-ligand coverage, whereas the LED performance changed significantly. In particular, a 2-fold difference in decyl-ligand coverage exhibited a 4-fold difference in electroluminescence (EL) turn-on voltage and a 17-fold difference in EL external quantum efficiencies. In addition, the LED performance was characterized by quantifying the relationship between ligand coverage, the number of bonding sites, and the surface areas of the ligands. At greater than 25% coverage, the total surface area of the decyl-ligands was significantly larger than that of a single SiQD, and when decyl-ligands and Si–O groups covered 50% of the surface, the insulation effect impaired the LED performance. Therefore, ligand coverage significantly affected the performance of SiQD LEDs. Although this study was limited to decyl-terminated SiQDs, the same method can be applied to other ligands to further improve LED efficiency of next-generation light sources in displays, lighting, and biomedical imaging.

Two-Photon Excitation Spectroscopy of Silicon Quantum Dots and Ramifications for Bio-Imaging.

Two-photon excitation in the near-infrared (NIR) of colloidal nanocrystalline silicon quantum dots (nc-SiQDs) with photoluminescence also in the NIR has potential opportunities in the field of deep biological imaging. Spectra of the degenerate two-photon absorption (2PA) cross section of colloidal nc-SiQDs are measured using two-photon excitation over a spectral range 1.46 < ℏω < 1.91 eV (wavelength 850 > λ > 650 nm) above the two-photon band gap Eg(QD)/2, and at a representative photon energy ℏω = 0.99 eV (λ = 1250 nm) below this gap. Two-photon excited photoluminescence (2PE-PL) spectra of nc-SiQDs with diameters d = 1.8 ± 0.2 nm and d = 2.3 ± 0.3 nm, each passivated with 1-dodecene and dispersed in toluene, are calibrated in strength against 2PE-PL from a known concentration of Rhodamine B dye in methanol. The 2PA cross section is observed to be smaller for the smaller diameter nanocrystals, and the onset of 2PA is observed to be blue shifted from the two-photon indirect band gap of bulk Si, as expected for quantum confinement of excitons. The efficiencies of nc-SiQDs for bioimaging using 2PE-PL are simulated in various biological tissues and compared to efficiencies of other quantum dots and molecular fluorophores and found to be comparable or superior at greater depths.

Designing Efficient Si Quantum Dots and LEDs by Quantifying Ligand Effects.

The impact of colloidal silicon quantum dots (SiQDs) on next-generation light sources is promising. However, factors determining the efficiency of SiQDs, such as the photoluminescence (PL) wavelength, PL quantum yield (PLQY), and the SiQD LED performance based on the type of ligand, ligand coverage, stress, and dangling bonds, have not been quantified. Characterizing these variables would accelerate the design and implementation of SiQDs. Herein, colloidal SiQDs were synthesized by pyrolyzing hydrogen silsesquioxane and their surfaces were terminated with 1-decene by either thermal hydrosilylation (HT-SiQDs) or room-temperature hydrosilylation using PCl5 (RT-SiQD). As a result, PL, PL-excitation, and ultraviolet-visible absorption spectra were similar, but their PLQYs were significantly different: 54% (RT-SiQDs) vs 19% (HT-SiQDs). To understand their similarities and differences, surface coverages (dangling bonds, Si-H (≡Si-H1, ═Si-H2, and -Si-H3), Si-O-Si, Si-C, Si-Cl) were determined. A core stress analysis established that a single ligand terminated to a SiQD bond site stretched the Si-Si bond length by 0.3%. From the two well-defined SiQDs, the PLQY and SiQD LED efficiency were attributed to four factors: low coverage of insulator ligands, the Cl ligand effect on radiative and nonradiative rates, negligible dangling bonds, and a SiQD core with low tensile stress. The PLQY of the RT-SiQDs in toluene was 80%. In addition, the 20× electroluminescence intensity difference of the LEDs originated from a 10× difference in current density and a 2× difference in Auger recombination. The concepts demonstrated here can be applied to further improve the PLQY and LED efficiencies of SiQDs with other ligands.

Hot-carrier radiative recombination through phonon confinement in silicon nanocrystals embedded in colloidal xerogel matrix

Colloidal suspension of free standing silicon/silicon oxide core/shell nanoparticles has been synthesized using a mechanochemical top-down approach. Quasi-mono-dispersed core size distribution of synthesized nanoparticles has been confirmed using different structural, morphological, and optoelectronic characterizations. Raman, continuous wave photoluminescence and time-resolved photoluminescence studies have been performed on synthesized colloidal nanoparticles in ethanol medium. Asymmetric broadening of the Raman peak (red shifted with respect to that of bulk silicon) has been observed. Intensities and positions of photoluminescence emission peaks are prominently dependent on excitation photon energy. Moreover, the photoluminescence decay time varies from sub-nanoseconds to tens of nanoseconds. The decay time also exhibits a strong dependence on the excitation wavelength, while the emission wavelength is kept unaltered. The abovementioned observations indicate the slow relaxation of photo-excited carriers in silicon quantum dots. This particular phenomenon takes place due to phonon mode discretization, which is further responsible for the radiative recombination of hot-carriers and consequent strong visible emission. The enhancement of hot-carrier lifetime in colloidal silicon quantum dots is the key requirement for the active material of the hot-carrier solar cell. Runny texture of the synthesized material inhibits practical device implementation; therefore, the synthesized nanoparticles have been embedded in the silica xerogel matrix. The impression of phonon mode confinement, in silicon quantum dots embedded in a hard matrix, has been observed, resulting in increased hot-carrier lifetime. The enhanced hot-carrier lifetime can lead to the realization of a silicon-based active material for the hot-carrier solar cell.

(Invited) Facile Formation of Luminescent Colloidal Silicon Quantum Dots from Porous Silicon

Silicon quantum dot is nanometer sized single crystalline silicon particles. It shows exotic luminescence properties such as size tuneablilty of luminescence band and quasi-direct electron-hole recombination resulting in efficient visible luminescence at room temperature, due to quantum confinement effects. Because of its environmental friendliness and low cost, the silicon quantum dot is a promising material for future light-emitting applications. Recently, a free-standing form of silicon quantum dot dispersed in solution, i.e., colloidal silicon quantum dots are intensively investigated due to their potential usefulness in solution based light emitting technology. The luminescence properties and solution dispersibility in silicon quantum dots are known to depend on the surface properties such as the coverage of surface termination and the kinds of surface terminated species.Various formation techniques for colloidal silicon quantum dot have been proposed, such as wet-chemical synthesis, plasma synthesis and high-temperature thermal processing. Pulsed-laser irradiation in liquid is one of the simplest top-down method for the formation of colloidal silicon quantum dots. By irradiating bulk crystalline silicon target with a high power pulsed-laser beam in liquid, silicon quantum dots are easily formed due to the ablation of silicon crystal and resultant condensation effect of ablated silicon at the liquid interface. An additional merit of this process is the easiness of surface termination control; because the nanocrystal surface formed by laser ablation actively reacts with molecules consisting of solution, one can control the surface termination by changing the kinds of solution. However, the control of the formed quantum dot size is still a challenging work and the size distribution of the quantum dots prepared by this process is usually wide.To overcome these issues, we developed a novel top-down pulsed laser irradiation technique for the formation of colloidal silicon quantum dots. We used an interconnected wire-nanostructure assembly, i.e., porous silicon (PSi), as a target material for the laser irradiation process, on behalf of bulk crystalline silicon target. PSi has beneficial properties for laser irradiation target such as low thermal conductivity and large surface areas. Because of these properties, the pulsed-laser irradiation of PSi target in organic solution allows us to generate the silicon quantum dots with relatively high preparation yields, and with various emission bands by changing irradiation conditions and the porosity of PSi. Furthermore, in our developed process, the formed dots are brighter than the original PSi target due to the stable organic terminations.Recently, we further improved our developed facile formation process of luminescent colloidal silicon quantum dots via porous silicon target. In the improved process, we employed electrochemically prepared PSi flakes, which purely consists of porous layer. The PSi flakes show extremely low thermal conductivity and heat capacity due to their intrinsic geometrical feature. Thus, even a low temperature thermal treatment as well as pulsed laser irradiation to PSi flakes in organic solution are efficiently converted porous silicon layer to colloidal silicon quantum dots. Furthermore, we succeeded in preparing silicon quantum dots dispersed in a polar and non-polar solvent depending on the ligands terminated on the dot surface by using different solutions. The present facile formation process of silicon quantum dots from PSi allows us to develop future top-down and low-cost mass production of luminescent colloidal silicon quantum dots.

White, blue and green emission from Si QDs derived from zinc incorporated porous silicon

This paper reports the tailored visible light emission (blue, green and white) traits of the colloidal silicon quantum dots (SiQDs) integrated with zinc (ZnSiQDs). These QDs were derived from the PSi etched with Zn powder. The ZnSiQDs was prepared via the photo-electrochemical etching process (40 mA/cm2 for 20 min) of the n-type Si wafer immersed inside the solution made from the hydrofluoric (HF) acid to ethanol (ET) volume ratios of 7:1 and 7:3 with the Zn powder (0.17 g) inclusion. The as-prepared samples (named as ZnSiQDs-7:1 and ZnSiQDs-7:3) were characterized to determine the influence of HF:ET on their structure, morphology and optical characteristics. The XPS analyses of the samples revealed the formation of ZnO. The FESEM images exhibited the growth of PSi layer of thickness 8.192 μm and 7.073 μm for the ZnSiQDs-7:3 and ZnSiQDs-7:1 samples, respectively. The EFTEM image analyses showed the nucleation of the ZnSiQDs of average diameter 2.7 nm (for ZnSiQDs-7:3) and 1.33 nm (for ZnSiQDs-7:1). The band gap energy for both samples estimated from the absorption spectra was 3.6 eV. The emission spectra of the studied ZnSiQDs under three different excitation wavelengths disclosed the intense visible peaks (blue, green and white) in the range of 385–700 nm. This observation was mainly ascribed to the effects of quantum confinement and ZnO surface layer on the SiQDs that minimized the silicon oxidation. It is established that by controlling the HF to ET volume ratio the visible luminescence of the ZnSiQDs can be customized, useful for diverse optoelectronic applications.

Visibly Transparent Solar Windows Based on Colloidal Silicon Quantum Dots and Front-Facing Silicon Photovoltaic Cells.

We demonstrate luminescent solar concentrators (LSCs) based on colloidal silicon quantum dots (SiQDs) as UV-selective fluorophores and coupled with front-facing silicon photovoltaic cells for the solar window application. The visibly transparent LSC composed of a thin layer of liquid SiQD suspension sandwiched between two thin glass slabs constitutes the windowpane, while strips of silicon photovoltaic cells with their front surfaces adhering to the LSC rear surface form the window frame. Furthermore, the LSC perimeter is surrounded by reflecting mirrors for preventing the fluorescence from leaking out through the edges. The SiQDs dispersed in 1-octadecene selectively absorb UV light and re-emit red fluorescence with quantum efficiency about 40%. Owing to the negligible overlap between the absorbance and photoluminescence spectra, the reabsorption effect is insignificant. The front-facing silicon photovoltaic strips located at the window frame can produce electricity by harvesting not only solar radiation but also the SiQD-generated fluorescence propagating from the windowpane. For the SiQD-LSC with the total light absorbing area equal to 12 cm × 12 cm and the reflecting mirrors tilted 45°, an overall power conversion efficiency of 2.47% under simulated sunlight can be obtained of which about 6% is contributed by the SiQD fluorescence. Meanwhile, the SiQD-LSC retains high spectral quality with average visible transmission and color rendering index through the windowpane equal to 86% and 94, respectively.

High-yield green fabrication of colloidal silicon quantum dots by low-temperature thermal cracking of porous silicon

We developed high-yield green fabrication of colloidal silicon quantum dots (QDs) only by low-temperature treatment of porous silicon (PSi) flakes in a reactive organic solution. Heat treatment at ∼50 °C of the PSi flakes in unsaturated hydrocarbon solution mixed with hydrofluoric acid (HF) yields the organically self-surface-passivated Si QD, which exhibit efficient photoluminescence compared with the conventional PSi. The production yield of Si QDs is enhanced by increasing the heat-treatment temperature and HF concentration, and the best results were obtained at ∼50 °C and 50%, respectively. The dispersibility of the prepared Si QDs in the solvents can be controlled by using appropriate organic function groups. The efficient transformation into Si QDs without waste is based on thermal cracking of weakly interconnected Si cores of PSi. Performing a practical clean process, the present result provides a promising route for overcoming limitations in the conventional colloidal QD preparation methods.

(Invited) Realizing Narrow Bandwidth Visible Photoluminescence from Colloidal Silicon Quantum Dots

Quantum dot light-emitting diodes (QD-LEDs) are an attractive alternative to organic light-emitting diodes (OLEDs) that have attracted attention for next generation optoelectronic devices due to their high colour-saturated photoluminescence, processability, and stability. Existing QD-LEDs often employ toxic or rare metals (e.g., Cd, Pb, In, etc.). Although colloidal silicon quantum dots (SiQDs) are an attractive alternative because of their abundance, biocompatibility and tailorable surface chemistry, tuning their luminescence throughout the visible spectrum can be challenging. Furthermore, SiQD emission shows a comparatively wide bandwidth that is a consequence of the inherent properties of the Si band structure. As a result the narrowing SiQD luminescence is not readily achieved by straightforward size selection methods used for other quantum dots. One alternative approach is to prepare optical cavities that preferentially select specific emission wavelengths using optical structures. This presentation will outline our investigations targeted on realizing bright, high quantum yield, narrow bandwidth luminescence from SiQD-polymer hybrids.

Silicon Quantum Dot Light Emitting Diode at 620 nm.

Here we report a quantum dot light emitting diode (QLED), in which a layer of colloidal silicon quantum dots (SiQDs) works as the optically active component, exhibiting a strong electroluminescence (EL) spectrum peaking at 620 nm. We could not see any fluctuation of the EL spectral peak, even in air, when the operation voltage varied in the range from 4 to 5 V because of the possible advantage of the inverted device structure. The pale-orange EL spectrum was as narrow as 95 nm. Interestingly, the EL spectrum was narrower than the corresponding photoluminescence (PL) spectrum. The EL emission was strong enough to be seen by the naked eye. The currently obtained brightness (∼4200 cd/m2), the 0.033% external quantum efficiency (EQE), and a turn-on voltage as low as 2.8 V show a sufficiently high performance when compared to other orange-light-emitting Si-QLEDs in the literature. We also observed a parasitic emission from the neighboring compositional layer (i.e., the zinc oxide layer), and its intensity increased with the driving voltage of the device.

Inverted Device Architecture for Enhanced Performance of Flexible Silicon Quantum Dot Light-Emitting Diode.

Here we report for the first time highly flexible quantum dot light-emitting diodes (QLEDs), in which a layer of red-emitting colloidal silicon quantum dots (SiQDs) works as the optically active component, by replacing a rigid glass substrate with a thin sheet of polyethylene terephthalate (PET). The enhanced optical performance for electroluminescence (EL) at room temperature in air is achieved by taking advantage of the inverted device structure. Our QLEDs do not exhibit parasitic EL emissions from the neighboring compositional layers or surface states of QDs over a wide range of driving voltages and do not exhibit a shift in the EL peak position as the operational voltage increases. Compared to the previous Si-QLEDs with a conventional device structure, our QLED has a longer device operational lifetime and a long-lived EQE value. The currently obtained brightness (∼5000 cd/m2), the 3.1% external quantum efficiency (EQE), and a turn-on voltage as low as 3.5 V are sufficiently high to encourage further developments of Si-QLEDs.

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.

Long-lived luminescence of colloidal silicon quantum dots for time-gated fluorescence imaging in the second near infrared window in biological tissue.

Boron (B) and phosphorus (P) codoped silicon quantum dots (Si QDs) are dispersible in polar solvents without organic ligands and exhibit photoluminescence (PL) in the first (NIR-I) and second (NIR-II) near infrared (NIR) windows in biological tissues due to the optical transition from the donor to acceptor states. We studied the relationship between the PL wavelength, lifetime and quantum yield (QY) of the colloidal solution and the composition of the starting material for the preparation. We found that the PL lifetime and the QY are primarily determined by the composition, while the PL wavelength is mainly determined by the growth temperature. By optimizing the composition, we achieved QYs of 20.1% and 1.74% in the NIR-I and NIR-II regions, respectively, in methanol. We demonstrate the application for time-gated imaging in the NIR-II range.

Charge-Transfer-Induced Photoluminescence Enhancement in Colloidal Silicon Quantum Dots

Charge transfer interaction between colloidal silicon (Si) quantum dots (QDs) and adsorbed molecules was investigated by means of photoluminescence (PL) spectroscopy. The molecule employed is tetrathiafulvalene (TTF), which has the highest occupied molecular orbital (HOMO) near that of Si QDs and can act as an electron donor to Si QDs. The energy difference between the HOMOs of TTF and Si QDs was controlled by changing the size of Si QDs. We found that the PL of Si QDs is strongly modified by the adsorption of TTF and that the PL change is reversible, that is, removal of TTF from a colloidal solution recovers the PL intensity. In the smallest Si QDs, where the HOMO level is expected to be 0.11 eV lower than that of TTF, a 2.5-fold enhancement of the PL was observed. The PL enhancement suggests that Si QDs having p-type behavior are compensated by electron transfer from TTF, similarly to the case of substitutional phosphorus doping in Si QDs. The observed size dependence of the PL enhancement factors sugge...