Impact of Anchoring Monolayers on the Enhancement of Radiative

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Impact of Anchoring Monolayers on the Enhancement of Radiative Recombination in Light-Emitting Diodes Based on Silicon Nanocrystals Batu Ghosh, Takumi Hamaoka, Yoshihiro Nemoto, Masaki Takeguchi, and Naoto Shirahata J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12812 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 4, 2018

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The Journal of Physical Chemistry

Impact of Anchoring Monolayers on the Enhancement of Radiative Recombination in Light-Emitting Diodes based on Silicon Nanocrystals Batu Ghosh,*,†,‡ Takumi Hamaoka,§ Yoshihiro Nemoto,§ Masaki Takeguchi,*,§ and Naoto Shirahata*,‡,ǁ,⊥

†

Department of Physics, Triveni Devi Bhalotia College, Raniganj, West Bengal, India

‡

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for

Materials Science (NIMS), 1-1 Namiki, Tsukuba, 305-0044, Japan §

ǁ

Transmission Electron Microscopy Station, NIMS, 1-2-1, Sengen, Tsukuba, 305-0047, Japan

Department of Physics, Chuo University, 1-13-27 Kasuga, Bunkyo, Tokyo 112-8551, Japan

⊥

Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-

0814, Japan

ACS Paragon Plus Environment

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ABSTRACT

In the current paper we provide direct evidence of a controlled structure of silicon nanocrystals (SiNCs). The photoluminescence quantum yields (PLQYs) are considerably enhanced by ligand exchange between the hydrogen atoms and hydrocarbon chains. To systematically study this phenomenon, we prepared SiNCs by thermal disproportionation of amorphous hydrogen silsesquioxane that was derived from triethoxysilane, which was followed by hydrofluoric etching and hydrosilylation of 1-alkenes. The estimated PLQY was 56% at maximum. The nearinfrared (NIR) PL spectra of the specimens can be tuned by accurately controlling their diameters to engineer the fundamental gap. Through a combination of X-ray diffraction (XRD), Raman spectroscopy, and scanning transmission electron microscopy (STEM), we elucidated that the alkyl monolayers provide an anchor that prevents the lattice distortion of the diamond cubic lattice of Si, thus inhibiting the creation of nonradiative channels. This anchoring effect is responsible for the high PLQYs. The emissions were sufficiently strong for the fabrication of NIR light-emitting diodes that operate in the first biological window (650–900 nm) where the light-absorption of water and the tissues including hemoglobin is minimal.


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1. INTRODUCTION The strong luminescence of diamond cubic silicon nanocrystals (SiNCs) is expected to be applied in a wide range of science, optoelectronics, and biomedical disciplines.1-10 Si is an attractive non-toxic alternative to heavy metals, ubiquitous in microelectronics, and abundant in the earth. Termination of the NCs with hydrogen atoms (ncSi:H) provides the most basic nonoxidized surface. The emission peak energy of ncSi:H is tunable from 600 to 1050 nm by controlling the size of the NCs. The ncSi:H behaves like a quantum dot (QD) of a direct bandgap semiconductor.11-14 Nevertheless, the reported photoluminescence quantum yields (PLQYs) of these NCs remain low (~15% at maximum). Surface modifications that enhance the radiative recombination and reduce the number of nonradiative recombination channels have intensified the PLQY to 65%.15-17 Such strong emissions have triggered new discussions on the optical performance of crystalline Si and inspired ceaseless efforts to develop light-emitting diodes (LEDs) with active ncSi layers.18-21 Surface modification is usually performed by hydrosilylation of 1-alkenes, but the success of the reaction largely depends on the experimental conditions. The reported PLQYs after surface modification vary from 20% to 65%. Previously, the alkyl monolayers bonded to the surface were believed to prevent the oxidation of ncSi, which creates surface defects that act as nonradiative channels. However, some papers have reported that surface oxide largely increases the PLQY.22-24 The misleading information resulting from this conflict prevents us from properly understanding the probabilities of intraband transitions in the excited states and inhibits the broad application of new ncSi-based light emitters that are fabricated through solution-processed approaches. In this study, to articulate the optical performance of ncSi:H as a light emitter before and after hydrosilylation of 1-alkenes, we compare the crystallographic structures of the two forms. For this purpose, we describe the synthesis of ncSi:H and of NCs capped with alkyl monolayers of


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different chain lengths. The novel feature of our synthetic protocol is its ability to separate the NCs with PLQYs exceeding 40% from those with poor emission performance. The purified SiNCs are characterized by XRD, Raman spectroscopy, and STEM. The high-performance ncSi specimens (PLQYs ≥ 40%) are adapted for solution-processed fabrication of LEDs that operate within the transparent window of soft tissues, thus minimizing the cutaneous phototoxicity to advanced light therapy technology.25-27 The emission wavelengths of the current-driven devices were tuned by controlling the NC diameter.

2. EXPEIMENTAL SECTION 2.1. Reagents and Materials Triethoxysilane (TES) was purchased from TCI chemicals. All the 1-alkenes such as 1octadecene, 1-decene, and 1-octene were purchased from Sigma-Aldrich, and used as received. 1,3,5-tris (Nphenylbenzimidazole-2yl) (TPBi) was purchased from Luminescence Technology Corp. and used as received. Electronic grade hydrofluoric acid (49% aqueous solution, Kanto Chemical), HPLC grade toluene, dicholrobenzene, ethanol, and methanol were purchased from Wako chemical. Poly-TPD, (LumTech), 1-octadecene, zinc acetate dehydrate, PEDOT:PSS solution (Sigma-Aldrich) were used as received. Water was purified and deionized using a Sartorius (arium 611 UV) water purification system. 2.2 Preparation of Nanocrystals The NIR light-emitting SiNCs were synthesized via a two-step process. In the first step, triethoxysilane (TES) was hydrolyzed by adding hydrochloric acid solution. In a typical, 5 mL of TES (6.0 g, 45 mmol) was added to a round-bottom two-neck flask equipped with a magnetic stir bar and stirred in Ar atmosphere using standard Schlenk techniques. A hydrochloric acid solution


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adjusted at pH 3 was added drop wise slowly to the TES in the flask. After some time, the transparent, colorless xerosol formed was filtered and washed to remove the acidic solution. The white gel obtained was dried in vacuum conditions at 100ºC for overnight to yield a white powder. The resultant powder was placed in a quartz crucible, transferred to a high-temperature tube furnace, and heated at the predefined temperature. A structural phase of the resultant xerogel was amorphous of hydrogen silsesquioxane as the results of characterization using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR).10,18 The amorphous hydrogen silsesquixane was dried and subjected to thermal disproportionation under a 5%/95% H2/Ar atmosphere. The ncSi size was tailored by tuning the temperature in the 1050°C–1100°C range. The reaction time at each temperature was controlled within 2 h. The resultant uniformly dark-brown powder was composed of SiNCs dispersed in a matrix of SiO2, which was determined in an XPS study.18 Next, the dark-brown powder was mortared into a fine powder. The oxide matrix was removed by stirring the fine powder for 1–2 h in an etching solution of hydrofluoric acid.10 The acidic solution was centrifuged at 15,000 rpm, and washed with ethanol, acetonitrile dichloromethane in this order. The precipitated products (ncSi:H) were immediately refluxed at 170ºC in At atmosphere for 10 min or less in either of three 1-alkenes (1-octene, 1-decene, or 1-octadecene). Each reaction batch yielded a transparent brown solution. It is noteworthy that the PLQYs of the specimens that require a reaction time of longer than 2hrs to reach the transparent state were low without exception. The unreacted 1-alkenes were removed by a vacuum evaporator. Finally, the chloroform solution of the alkane-capped NCs was purified and separated by gel permeation chromatography (GPC). The GPC-treated specimens were dried under vacuum conditions, and stored in Ar atmosphere prior to use for spectroscopic characterization and device fabrication. Interestingly, the NCs terminated with


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octane (ncSi-Oct) or decane (ncSi-De) behaved as a non-viscous solid powder, while that terminated with octadecane (ncSi-OD) was slightly viscous possibly because of the long length of the hydrocarbon chains than in the other terminations. 2.3. Characterization X-ray powder diffraction (HT-XRD, RINT-TTR II and Reactor X, Rigaku, Japan) was used to characterize the major crystalline phases of the specimens, and to estimate the diameters of the samples from the diffraction linewidths of each phase using Scherrer equation. For the STEM observation, the sample was prepared by drop-casting the diluted chloroform solution of NCs on the copper grid covered with ultrathin amorphous carbon film. The observation of the lattice fringes of NCs was on the JEOL JEM-ARM200F with STEM mode operating at 200 kV. This offers an unprecedented opportunity to probe structures with sub-Angström resolution. In the STEM observation, the bright-field and the dark-field images were acquired with in-line and HAADF detectors, respectively. A low-pass filter was applied to the image for noise reduction. Raman measurements were carried out on NC films deposited from dichloromethane colloids on a gold-coated substrate. Raman spectra at a single excitation wavelength (532 nm) were on a slitscanning Raman microscope (RAMAN-11; Nanophoton, Osaka, Japan). The optical absorption of the specimens was studied by measuring the diffuse reflectance spectra (JASCO V-650 spectrophotometer). Optical properties of all the specimens were measured using their solid forms as powder unlike oily-like Si nanoparticles. Also the use of powder samples was useful to avoid solvent effect. PL and PLE measurements were carried out using a modular double grating Czerny–Turner monochromator and an iHR 320 emission monochromator (1200 lines/mm of gratings) coupled to an InGaAs Hamamatsu photodetector on a NanoLog Horiba Jovin Yvon spectrofluorometer with 450W xenon arc lamp. The spectral resolution of the system is around


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0.3 nm. The time-resolved PL decay curves were acquired with the same NanoLog Horiba Jovin Yvon spectrofluorometer, using a pulsed spectral LED of 370 nm emission. The absolute PLQYs were measured at room temperature using the QY measurement system C9920-02 from Hamamatsu Photonics Co. Ltd with a 150 W xenon lamp coupled to a monochromator for wavelength discrimination, an integrating sphere as a sample chamber, and a multichannel analyzer for signal detection. We used a standardized integrating-sphere method that is widely known to estimate an absolute value of PLQY, for its accuracy. The use of quarts cells just after strict washup followed by drying under vacuum has provided precise measurement. In contrast, the use of contaminated quartz cell (even the viewless contaminations physisorbed at the quartz surface) caused measurement errors of 30% at maximum. 2.4. Device Fabrication and Device Characterization A 150 nm thin film of indium tin oxide (ITO) was uniformly sputtered on a soda-lime glass. A resistivity of the ITO film was measured to be less than 15 Ω/square. The ITO-coated glass substrates were first etched using HCl and Zn dust in a narrow strip. Then they were cleaned by ultrasonic agitation with a nonionic detergent, followed by in ethanol, in acetone and isopropanol and finally rinsed with 18.2 MΩ/cm water. After drying, the ITO-glass substrates were transferred to a reactive ion etching (RIE) chamber for plasma treatment, treated for 10 min under a pressure of 1 m Torr of nitrogen and oxygen mixture. Then PEDOT:PSS solution was first spin-coated onto plasma-treated substrates at a speed of 5000 rpm for 45 sec. Afterwards the PEDOT:PSS-coated substrates were loaded into Ar-filled homemade chamber, and baked at 140 °C for 30 min to eradicate any remaining solvent before the deposition of subsequent organic and ncSi layers. A ~20 nm of ncSi-De layer was coated on the PEDOT:PSS surface by spin-coating the ncSi-De solution (10 mg/ml) for 1 min at 800 rpm. The films were dried in Ar


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atmosphere at 100 °C for 1.5 hr. TPBi and aluminium (Al) were deposited as an electron transportation layer (ETL) and a top electrode cathode by vacuum evaporation in this order. The devices were characterized in an ambient condition. Current-voltage (I-V) characteristics were recorded with a computer interfaced Keithley 2425 electrometer, luminous output of the LEDs was measured with a calibrated silicon photodiode (Hamamatsu S1336-8BQ) coupled to a another computer interfaced Keithley 2400 electrometer in ambient air. Contact to the top Al cathode was made using Ag paste. EL spectra were measured using a computer interfaced Ocean Optics USB2000 fiber spectrometer. To determine the photometric brightness (cd/m2) of the LED, the diode output power was first measured silicon photodetector that was directed at a fixed distance toward the ITO glass side of the LED. The LED luminance (brightness) was then calculated from the known portion of the forward emission and the LED output spectra.

3. RESULTS AND DISCUSSION 3.1. Preparation of SiNCs with Different Terminations Figure 1 shows the XRD patterns of ncSi:H specimens with different average diameters. Using Bragg’s law, each peak was indexed and associated with an interplane spacing (d-spacing) of the diamond cubic lattice structure. The absence of other diffraction lines indicates the absence of dioxonium hexafluorosilicate or any other crystalline structure.10 The diameters of the NCs were determined by Debye-Scherrer broadening, which is valid for estimating diameters at the single nanometre scale.11 Interestingly, close examination reveals that as the diameter decreases, the peak not only broadens but also slightly shifts toward small diffraction angles. To the best our knowledge, such a size-dependent shift of the diffraction lines has not been previously reported. This shift might reflect the expansion of the diamond cubic cells, thus causing changes in the unit cell dimensions, symmetry, and lattice (i.e., changes in the crystal plane spacing).28,29


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The optical properties were determined on solid-powder specimens to avoid the solvent effect. The combined UV–Vis absorbance and scattering were studied via Kubelka–Munk analysis. Figure 2a plots the measured results for each ncSi:H specimen. As the diameter decreases, the peaks of the absorbance/scattering ratio are blueshifted (dotted line in Figure 2a). The PL spectra (measured on the same specimens) exhibit narrow emission lines with no long emission tails (Figure 2b). The specimens are continuously tunable over the 700–900 nm range, which is called the first biological optical window. As predicted, the specimens demonstrate a large Stokes shift between their absorptions and emissions. Interestingly, as the ncSi:H diameter decreases from 3.6 to 2.1 nm, the absorption maximum slightly increases from 3.14 to 3.27 eV. This small change contrasts with the large shift of the PL peak energy from 1.48 to 1.72 eV. The absolute PLQYs of the same specimens were measured by the standardized integrating-sphere method;30 these values are summarized in Table 1. For the ncSi:H specimens, it is reasonable to see the decreasing PLQY with an increase in the size of the NCs possibly because of a weakened effect of the quantum confinement of the photoexcited carriers.

3.2. Role of Anchoring Monolayers for Intensifying the PLQYs In the hydrosilylation of 1-decene, a covalent hydrogen–silicon bond in the NCs is replaced by a covalent carbon–silicon bond. Consequently, the PLQYs dramatically increased by up to 56% regardless of the NC diameter (see Table 1). Similar phenomena were observed in the specimens of NCs with alkyl monolayers of different chain length. To relate this dramatic increase to the NC structure, we compared the crystallographic and vibrational properties of the ncSi:H and ncSi-De specimens. Figure 3 shows the XRD patterns of the 2.1-nm ncSi:H specimens before and after alkylation. The 3.6-nm ncSi:H specimen served as a standard for bulk-crystalline Si. It is noteworthy that the intensity peaks of the 2.1-nm ncSi:H shifted to small diffraction angles, while those of the ncSi-De specimen were unchanged from the standard diffraction angles. This comparison suggests that the alkylation of surface Si atoms differs from hydride-termination and maintains the structural parameters (lattice periodicity, interatomic distances and number of neighbors) at their values observed in the bulk-crystalline structure. Next, the ncSi:H and ncSi-De specimens of different diameters were characterized by Raman spectroscopy, which reveals differences in their vibrational properties. In bulk Si, only the optical phonons at the center of the Brillouin zone (∆q = 0) participate in the first-order Raman


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scattering process. The crystal momentum is conserved to generate a sharp peak at 521 cm−1 in a symmetric vibration region. However, momentum is not necessarily conserved in the nanometre size range. The relaxation of the momentum selection rule allows the occurrence of Ramanactive modes away from the Brillouin zone center. The offset is limited to ∆q = ~π/d (where d is the SiNC diameter). Consequently, the peak broadens asymmetrically and redshifts in a sizedependent manner. Figure 4 shows typical Raman spectra of the ncSi:H and ncSi-De specimens with different diameters. As predicted, the Raman peaks of the ncSi-De specimens shift to a low frequency, broaden, and develop an asymmetric band shape as the NC diameter decreases. However, the ncSi:H band shape as a function of diameter evolves in an entirely different manner. The asymmetry in the Raman profile is evident for 3.6-nm ncSi:H (consistent to the above manner), but the peaks of the other two specimens (2.1-nm and 2.6-nm ncSi:H) remain centered at 495 cm−1 and retain symmetric spectral features. These properties violate all trends of theoretical models that explain the band-shape modification of the Raman peak with a decrease in NC size.31-35 The different terminal groups also influence the magnitude of the Raman shift. For ncSi, the frequency downshift of the Raman peak with a decrease in diameter can be explained by the correlation length model.33,36 Once the Raman shift is known, the crystalline diameters are calculated as follows: a ∆ϖ (D) = −A D

γ

(1)

where ∆ω(D) is the Raman shift of ncSi as a function of diameter (D), and a is the lattice constant of diamond cubic Si (0.543 nm). A = −97.462 cm−1 and γ = 1.39 are fitting parameters that describe the phonon confinement in nanometric spheres of diameter D.33 Figure 5 plots the Raman spectral shifts as functions of D for the ncSi:H and ncSi-De specimens (closed rhombi and closed circles, respectively). The correlation length model (open squares) satisfactorily predicts the ncSi-De trends, but it cannot describe the frequency downshift of the Raman peaks of ncSi:H. A Raman shift of ~495 cm-1 has also been reported by several groups. Kůsová et al. proposed that tensile strain caused by deformation of the diamond cubic Si lattice modifies the Raman signal by 495 cm−1 because strain influences the phonon energy.37 Other theoretical and experimental studies have supported the assumption that distorted crystalline structures hamper


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the long-range ordered arrangement of Si atoms, thus generating short-range ordered atomic configurations.35,38-40 Thus, one may reasonably infer that the large shift (~495 cm−1) occurs by an emergent intermediate range that covers both long-range ordering of the crystalline state and short-range ordering that approximates amorphous configurations.15 Figure 6 shows the Raman spectra of three specimens of 2.1-nm NCs terminated with alkyl monolayers of different chain lengths. Elongating the methylene chain does not influence the size-dependent evolution of the vibration properties including Raman shift (as indicated by the vertical dotted line) and spectral broadening and shapes, thus suggesting that the correlation length model adequately explains the band-shape modification of the Raman peak even for NCs with different hydrocarbon chain lengths. The STEM observations directly confirm the significantly different crystalline structures of ncSi:H and ncSi-De. A bright field (BF) image of STEM was simultaneously recorded with high-angle annular dark-field (HAADF) images. Figures 7a and 7d are BF-STEM images of ncSi:H and ncSi-De, respectively. The round (~2.1 nm wide) particles are the NCs and the dark spots represent the atomic columns of Si. Figure 7c shows the variation of the (220) lattice spacing in ncSi:H within the area enclosed by the green rectangle in the accompanying image. From the line profile of the image contrast, the (220) spacing was determined as 0.17, 0.19, and 0.22 nm. The spacing of 0.19 nm agrees with the theoretical (220) lattice spacing of diamond cubic Si. The deviation from the expected value could indicate the occurrence of lattice distortion. The XRD angles calculated from the deviant lattice spacing (0.17 and 0.22 nm) are covered by the broadened diffraction lines of the 2.1-nm ncSi:H specimen (see Figure 1). Such a lack of structural coherence has not yet been reported in ncSi:H of 5 nm or larger.41-43 We suggest that the 2.1-nm NCs are sufficiently small to lose their structural periodicity, thus increasing the level of random distribution, broadening the distribution of bond lengths, and angularly distorting the lattice. In contrast to ncSi:H, the lattice edges and fringes of the ncSi-De specimen cannot be located precisely as the aliphatic capping ligands are obscured by the imaging contrast originating from the amorphous carbon film that covers the entire surface of the STEM copper grid (see Figure 7d). The imaging contrast of this specimen was improved under HAADF-STEM mode, which has a Z-contrast sensitivity (Figure 7e). In the enlarged image (Figure 7f), the (200) lattice


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spacing of ncSi-De was determined as 0.27 nm across the entire NC to confirm a {200} facet termination. The d-spacing of 0.27 nm is the expected (200) lattice spacing of bulk Si. The clear lattice symmetry of ncSi-De indicates that the terminal hydrocarbon chains preserve the structural coherency of the diamond cubic lattice throughout the entire nanocrystalline Si irrespective of the diameter. However, when the outermost Si atoms are terminated with hydrogen atoms, the lattice distortion is significant and the NCs become round in shape (see Figure 7a-c). Therefore, one may reasonably conclude that the ncSi:H structure contains many trap states for photogenerated carriers. Obviously, the lattice expansion can break the covalent Si–Si bonds, thus generating defects such as dangling bonds, atomic vacancies, and highly distorted structures. These local energy minima trap excited electrons and holes, thus increasing the probability of nonradiative recombination. From the Raman spectra and STEM observations, we infer that the alkyl monolayers behave as anchors that maintain the coherent diamond cubic structure throughout the SiNC. For the alkylated NCs, because of maintaining the crystallographic symmetry that does not provide the local energy minima between the fundamental energy gaps, this anchorage effect drastically reduces the chances of creating trap states that capture electron–hole carriers.

3.3. Device Fabrication and Characteristics The structure of LED devices that work in the biological first window is illustrated in Figure 8a. The layers are constructed as follows: indium tin oxide (ITO)-coated soda glass/poly (ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS, 40 nm)/nSi-De/2,2′,2″-(1,3,5benzenetriyl) tris-(1-phenyl-1Hbenzimidazole) (TPBi, 40 nm)/Al (120 nm). The Al cathode and the TPBi electron transportation layer (ETL) were deposited by vacuum thermal evaporation; all other layers were sequentially deposited on the ITO surface in the above order by spin-coating. The PEDOT:PSS layer was spin-coated onto the UV–ozone treated ITO substrates and dried at 140°C under an Ar atmosphere for 15 min. The ncSi-De layer was deposited on top of the ITO/PEDOT:PSS layer by spin-coating dispersed ncSi-De (20 mg/ml in toluene) at 2000 rpm for 60 s and was subsequently dried at 110°C under an Ar atmosphere for 60 min. The optical and electrical properties of the devices were characterized under ambient conditions without using an integrating-sphere system. Under the present conditions for device evaluation, we compromised a risk of lower external quantum efficiency than the real value. These multilayer ncSi-based


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devices can be potentially fabricated via high-throughput solution-processing techniques such as inkjet printing. Figure 8b shows the energy levels in the corresponding device architectures. With the exception of the ncSi-De active layer, the highest occupied molecular orbital (HOMO) energy levels of the transport layers were taken from the literature.18,19 The HOMO and lowest unoccupied molecular orbital (LUMO) energies in the active layer were estimated by correcting those of bulk Si for the quantum confinement effect.44 According to the energy level diagram, the TPBi (with a HOMO of ~6.3 eV) establishes a valence band offset at the ncSi-De/TPBi interface; this helps to confine holes within the ncSi-De layer, thus improving the charge recombination efficiency. PEDOT:PSS efficiently injects the holes from the ITO anode into the ncSi-De layer. Because the ionization potentials of the ncSi and PEDOT:PSS HTL layers are very similar (~5.3 and ~5.2 eV, respectively), another layer for hole transportation was not needed. The electrons and holes injected from each electrode are expected to recombine radiatively inside the ncSi layer to emit electroluminescence (EL). The current–voltage (I–V) characteristics of three devices with different ncSi-De diameters and PL peak energies are plotted in Figure 9. As shown in Figure 9a, the turn-on-voltage increases with a decrease in the ncSi diameter. This suggests that increasing the ncSi size increases the conductivity of the device possibly by smoothing the injection of electrons from the Al electrode (~4.3 eV) into the ncSi-De layer. This result is consistent with the results of previous theoretical studies.45,46 Charge carrier transport in QD-based devices (including multilayered ncSi-based devices) has usually been described by the space-charge limited (SCL) conduction model.18 To investigate the current conduction mechanism in our devices, we calculated the slopes m of their J–V characteristics plotted on a double-logarithmic scale (Figure 9b). The slope is the exponent in the power law relationship J ∝ Vm and is related to the distribution of trap states in a semiconductor.47 Carrier transport through NCs and organics is known to be influenced by defects in the discrete states and at the interfaces between two device layers. In the proposed device, inorganic ncSi is coated with an insulating organic monolayer, i.e., decane molecule. Therefore, the electrodes supply more carriers than the number of carriers that is actually transported between the layers; thus, the device operates in the SCL regime. The log–log plots in Figure 9b can be fitted to three regimes bounded by Ohmic conductance (J ∝ V), charge-trapfree SCL conductance (J ∝ V2) and charge-trap-assisted conduction (J ∝ Vm > 2). Under a low


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applied bias (V < VTR, where VTR is the minimum voltage that induces carrier transition), the J–V characteristics follow Ohm’s law, thus implying that the density of thermally generated free carriers inside the films is larger than that of the carriers injected from the electrodes. When the charge conduction mode transits from Ohmic conduction to SCL conduction, the carrier transit time becomes equal to the dielectric relaxation time and the applied voltage reaches VTR. In the third regime (when the applied voltage exceeds 3.4 V), the estimated slope m ~ 3.2 indicates entry into a conduction mode assisted by the exponentially distributed trap states. In this bias regime, the conduction mode reverts to the SCL behavior without traps (m ~ 2), thus suggesting that although the trap states are filled during SCL conduction, the traps are not saturated.48 Figure 9c plots the evolving photodiode current as the applied voltage increases in the three devices. The estimated turn-on-voltages in the red, deep-red, and NIR devices were 2.5, 2.3, and 2.1 V, respectively. Interestingly, decreasing the size of ncSi-De in the device lowered the photodiode current density at high applied voltages, but it did not significantly affect the turn-onvoltage for device operation. This is possibly due to the use of the emission layers of a same chemical composition for each device. This assumption is supported by the previous study where the turn-on-voltages of QD-based LEDs are dependent of chemical composition of the QD even in a same device architecture.49 The observation of the similar turn-on-voltages is attributable to the size dependence of the emissions, while the small differences in the turn-on-voltages shown in Figure 9c could be explained by the narrowing fundamental energy gap between HOMO and LUMO as the ncSi size increases. Notably, the electron–hole carriers injected from each electrode recombine in the ncSi-De layer. The performance of solution-processed ncSi-based LEDs has greatly improved since their introduction in the literature. However, the high turn-on-voltage, low external quantum efficiency (EQE), and non-negligible parasitic EL emission from the adjacent conjugated organic layers or surface-trap states of the NCs are ongoing problems.21,50 The optical performances of the ncSi-based devices are summarized in Figure 10 and Table 2. The EL spectra shown in result from the recombination of the injected electron–hole carriers in the NCs, which explains the size-dependency of the EL peak energy. Interestingly, the EL peak positions are blueshifted by ~20, ~10, and ~20 nm from the initial PL spectra in which they are centered at 720, 770, and 840 nm, respectively (see Table 2). This shift behavior is opposite to the previous case where the redshift of EL spectral peaks was observed for the QD-based devices


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with emission layers of CdSe/CdTe core/shell structures.51 Figure 10b plots the EQE, which is estimated as the EL output from the photocurrent measured by a photodetector, as a function of driving current density. The EQE was estimated as follows:

EQE (%) =

[ூሺ௣௛ሻିூሺௗሻ]×௤ ோ×ூ×ாሺ௣௛௢௧௢௡ሻ

݃ × 100 (2)

where I(ph) is the photocurrent detected by the photodiode (Hamamatsu S1336-8BQ), placed just below the EL device. In this setup, we can detect a limited number of photon. Moreover, the measurements were not made on integrated spheres. Therefore, the number of emitted photons was higher than the number detected by the photodiode, thus leading to underestimated EQEs. In Eq. (2), I(d) is the dark current of the idle photodiode, I is the device current, R is the responsivity of the photodiode, q is the electronic charge, E(photon) is the photon energy, and g is the configuration factor of our measurement setup, which was estimated as 0.3. The peak EQE values of the red, deep-red, and NIR emissions were estimated as 0.2%, 0.23%, and 0.22%, respectively. As shown in Figure 10a, there was no parasitic emission from the neighboring fluorescent organic layers (poly-TPD) or the surface states of ncSi owing to the facilitated electron–hole carrier injection into the ncSi emissive layer from the HTL and ETL and possibly owing to the good electron–hole balance. Figure 10c displays the LED performance in terms of luminance voltage. The luminances of the red, deep-red, and NIR LEDs were maximized at 5.4, 4.5, and 4.4 cd/m2, respectively. 4. CONCLUSION We discussed the mechanism by which thermal hydrosilylation of 1-decene considerably enhances the PLQYs of SiNCs. The maximum PLQY of the resultant NIR-emitting ncSi specimens was 56%. The fabricated specimens were used as the emission layers in solution-


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processed LED with EL performances in the biological optical window. The PL spectra of ncSi specimens with average diameters of 2.1, 2.7, and 3.6 nm were centered at 720, 770, and 840 nm, respectively. A combination of Raman spectroscopy, STEM, and X-ray powder diffraction verified that as the size of the NCs decreased, the outer surface atoms of ncSi:H were distorted to compensate the crystal strain induced by the high surface-to-volume ratio. However, the surface monolayers of alkyl-terminated ncSi provided a molecular anchor that prevented lattice distortion, thus yielding the unformed local energy minima traps to result in the PLQY. The proposed device architecture adapted for NIR LEDs exhibited no parasitic emission, thus solving a persistent problem in ncSi-based devices. Like the PL peaks, the EL spectral peak energies depended on the ncSi diameter. Although the device performance must be further improved for practical use, tunable NIR emission in ncSi LEDs opens new possibilities in future biomedical and healthcare applications.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: 81-29-859-2743. *E-mail: [email protected]. Tel: 91-7797-724233. *E-mail: [email protected]. Tel: 81-029-859-2486

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This study was supported by the WPI-Program, JST A-step (AS282I006e), Kakenhi and Sumitomo Foundation


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(111)

(220)

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(311)

2.1nm

2.6nm 3.6nm

20

30

40

50

60

2θ (degrees) Figure 1. XRD patterns of 2.1 nm, 2.6 nm and 3.6 nm ncSi:H specimens, respectively. The dotted lines shows the theoretical position of the diffraction lines for (111), (220) and (311) planes of bulk-crystalline Si.


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Figure 2. (a) Plots of F(R∞)2 versus wavelength for the ncSi specimens. F(R∞) of powder form is the Kubelka-Munk function, with F(R∞) = (1 - R∞)2/2R∞. (b) PL spectra for the corresponding ncSi:H specimens of 2.1 nm, 2.6 nm and 3.6 nm diameter.


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(111) (220) (311)

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.1nm ncSi-De

2.1nm ncSi:H

3.6nm ncSi:H 20

30

40

50

60

2θ (degrees)

Figure 3. XRD patterns of 2.1 nm ncSi:H specimens before and after hydrosilylation of 1-decene. The diffraction pattern of 3.6 nm ncSi:H was shown as a standard.


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Figure 4. Raman spectra of the ncSi:H and the ncSi-De specimens of different average diameters, measured with excitation at 532 nm.


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Figure 5. Raman shift of the peak positions as a function of diameter of ncSi. The size-dependent redshift of Raman peak positions is calculated by the correlation length model (open squares), respectively.1 Plots of Raman peak positions for our ncSi:H specimens with different average diameters were shown by the closed circles (ncSi-De) and the closed rhombus (ncSi:H).


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Figure 6. Raman spectra of three specimens of 2.1-nm SiNCs terminated with alkyl monolayers of different chain lengths such as octadecyl- (OD), decyl- (De) and octyl-monolayers (Oct).


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Figure 7. STEM images of the 2.1 nm ncSi:H specimen before and after hydrosilylation of 1-decene. From left to right: (a,d) BF- and (b,e) HAADF-STEM images. (c,f) The magnified images and line profiles of the green-rectangles for each NC pointed by the arrows in the images (a) and (e). A low-pass filter was applied to acquire all the images.


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Figure 8. (a) Schematic of layers in the NIR light emitting diodes with ncSi-De of different diameters. (b) Proposed energy level diagram under zero applied bias for devices containing emissive ncSi-De.


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Figure 9. (a) Device I-V characteristics, (b) log–log plots of the device current density versus voltage characteristics and (c) photodiode I-V characteristics for the three ncS-based devices in Figure 8.


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Figure 10. Performance of the light emitting diodes with emission layers of ncSiDe of 2.1 (λem= 700 nm), 2.6 (λem= 760 nm) and 3.6 nm (λem= 820 nm). (a) EL spectra of the three devices operated at 4V. Photographs are taken through the ITO anodes and show EL colors of the 700 (red) and 760 nm (dark-red) emitting devices. (b) EQE-current density and (c) Luminance-current density characteristics for the devices.


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Table 1. Summary of structural and optical characterization results. Diameters of the NCs were estimated from Scherrer analysis of XRD line broadening of the ncSi:H specimens. Film forms of ncSi:H and ncSi:De specimens were prepared by drop-casting each of them to the surface of quartz glass substrates, and used for optical characterization.


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Table2. Summary of the Optical and Electrical Properties of ncSi-De and LEDs


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Table of Contents (TOC) Graphic


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Impact of Anchoring Monolayers on the Enhancement of Radiative

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