Thickness-Dependent Full-Color Emission Tunability in a Flexible

Apr 2, 2014 - (1, 2) Materials with tunable colors from the same luminescent component are highly desirable. ... confined inside the nanosized pores o...
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Letter pubs.acs.org/JPCL

Thickness-Dependent Full-Color Emission Tunability in a Flexible Carbon Dot Ionogel Yu Wang,†,∥ Sergii Kalytchuk,†,‡,∥ Yu Zhang,†,§ Hengchong Shi,†,⊥ Stephen V. Kershaw,† and Andrey L. Rogach*,† †

Department of Physics and Materials Science and Centre for Functional Photonics (CFP), City University of Hong Kong, 88 Tat Chee Avenue, Kowloon, Hong Kong SAR ‡ Clean Energy and Nanotechnology (CLEAN) Laboratory, School of Energy and Environment, City University of Hong Kong, Shatin, N.T., Hong Kong SAR § State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, P. R. China ⊥ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China S Supporting Information *

ABSTRACT: Multicolor luminescent materials are of immense importance nowadays, while it still constitutes a challenge to achieve luminescence color tunability, transparency, and flexibility at the same time. Here we show how ultrasmall carbon dots (CDs) fluorescing strongly across the visible spectrum can be surface functionalized and incorporated into highly flexible hybrid materials by combination with ionic liquids within silica gel networks to form CD-ionogels with properties promising for fabrication of flexible displays and other optical technologies without the use of any toxic materials. We demonstrate how the emission from such hybrid materials can be tuned across a large range of the Commission Internationale de l’Enclairage (CIE) display gamut giving fullcolor performance. We highlight how the rich ladder of emissive states attributable to organic functional groups and CD surface functionalization supports a smooth sequential multiple self-absorption tuning mechanism to red shift continuously from blue emitting nπ* transitions down through the lower energy states. SECTION: Physical Processes in Nanomaterials and Nanostructures

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have been prepared upon the surface passivation with functional molecules, such as oligomeric poly(ethylene glycol) diamine (PEG1500N),17 branched poly(ethylenimine) (BPEI),21 and organosilane.22 Recently, there have been a few reports on the dramatic luminescence enhancement of CDs through a chemical reduction, which increases the number of sp3-type carbon defects resulting in the conjugation of the defect areas.23,24 Moreover, CDs possess a number of further advantages, such as broadband optical absorption, good chemical stability, and low toxicity.15 The surface of CDs can easily be functionalized, which renders them useful components for various luminescent hybrids.25,26 Ionic liquids (ILs) are room-temperature molten salts, composed of large organic cations and various inorganic (or organic) anions, with almost unlimited structural and functional variations.27 Their properties, such as negligible vapor pressure, thermal stability, high ionic conductivity, and wide electrochemical potential window, make ILs useful for the develop-

ulticolor luminescent materials have attracted considerable attention owing to their potential applications in multiplexed biological labeling, light-emitting displays, and the next-generation lighting sources.1,2 Materials with tunable colors from the same luminescent component are highly desirable.3,4 Excitation-dependent photoluminescence (PL) has been recently demonstrated for carbon dots (CDs),5,6 making them promising candidates for multicolor sensing,7 bioimaging,8,9 light-emitting diodes,10,11 and patterning.12 Although the origin of excitation-dependent PL is still a matter of debate, several mechanisms have been suggested to explain this optical property of CDs, such as the size distribution of CDs (quantum-confinement effect),13,14 different emissive traps on the CD surface,15−17 and radiative recombination of excitons.18 Owing to a high concentration of C-sp2 hybridization shown in almost all CDs, the band-gap transitions referred to conjugated π-domains, and some complex origins connected to the defects in the graphene structures may also be considered.19,20 It should be mentioned that the surface passivation associated with the synthetic method is essential for the remarkable improvement of the photoluminescence quantum yield (PL QY) of CDs.15,17 A large number of CDs with high PL QYs © 2014 American Chemical Society

Received: March 15, 2014 Accepted: March 31, 2014 Published: April 2, 2014 1412

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Figure 1. (a) Schematic drawing of a carbon dot with multiaminosilane-functionalized surface. (b) True-color photograph showing emission of SiCDs under varying excitation wavelength, as indicated. (c) Absorption and excitation spectra of Si-CDs colloidal solution. The excitation spectrum has been constructed from the peak values of each of the emission spectra (panel d); that is, the emission wavelength was effectively shifted from one excitation point to the next to track the shift in the PL spectrum with excitation. The inset presents an enlarged absorption spectrum in the 400−700 nm range. (d) Photoluminescence spectra recorded for excitation wavelengths from 320 to 540 nm in 20 nm increments. Inset: Normalized PL spectra at the corresponding excitation wavelengths. (e) Excitation-dependent PL spectra in panel d were fitted to triple Gaussian functions (as shown in Figure S1 in the Supporting Information), and the resulting peak positions are shown for each excitation photon energy case. The black horizontal bars show the excitation energies, and bars colored blue, green, red, dark red, and cyan show the resulting emission peak energies. The length of each of the latter is scaled to show the relative amplitude of each of the three fitted Gaussians in each group. Also shown for reference are the energies of the π−π* and n-π* transitions from the absorption spectra (panel c) with hatched areas corresponding to the ranges between the absorption peak half maxima of the latter. The spectral extent of the absorption assigned to surface states is not shown here. The dashed curve shows the PL QY measurements for some of most prominent cases.

ment of a variety of hybrid materials.28−30 The challenge of immobilization of ILs in solid matrices has been addressed by the use of ionogels, which consist of an IL confined inside the nanosized pores of a transparent crack-free silica monolith.31,32 In the silica sol−gel process, ILs can act as nonvolatile chemical component owing to their negligible vapor pressure, which otherwise would cause problems of shrinkage and subsequent matrix collapse during the formation of gels.33 Ionogels exhibit advantages such as good transparency, outstanding ionic conductivity performance, stability in water and various organic solvents, and the ability to be easily shaped into coatings, rods,

and pellets. Luminescent ionogels can be obtained by the inclusion of ILs containing luminescent species in silica34,35 or polymers.36,37 Flexible, highly transparent luminescent ionogels containing lanthanide ions have been recently reported,38 with the emission color fine-tuned by the amount and type of lanthanide and by varying the excitation wavelength. Here we have created a flexible, free-standing, and transparent ionogel by combination of a carboxyl-functionalized IL {1-(3-carboxypropyl)-3-butylimidazolium bromide, [CarbBim]Br} with organosilane-functionalized carbon dots (SiCDs) (Figure 1a). Using just a single excitation wavelength, 1413

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wavelength-dependent, we have constructed an excitation spectrum from the peak intensity values of the respective sequence of emission spectra at different excitation wavelengths so that it corresponds to the trajectory of the emission peak position of a PL excitation/emission map, referred to as the excitation coordinate. The trajectory of the PL peak and the corresponding PL excitation spectra for each characteristic PL peak (λem = 450, 550, and 610 nm) are shown in Figure S4 in the Supporting Information. Figure 1b,d shows true color photographs and PL spectra of Si-CDs with a broad spectral coverage from 450 (blue emission) to 610 nm (orange emission) dependent on the excitation wavelength, as has been previously reported.12,43,44 The excitation-dependent emission spectra shown in Figure 1d were further analyzed by fitting the complex emission peaks to multiple Gaussian functions (as shown in Figure S1 in the Supporting Information). In each case, very good fits of the spectra could be obtained by decomposing them into three simple Gaussian peaks. The photon energies of these three fitted peaks, for each excitation energy, are shown schematically in Figure 1e. The Figure also shows the excitation energies, and the relative amplitudes of each of the emission peaks in each group are indicated by the horizontal lengths of the colored bars. Alongside the PL and excitation energies, Figure 1e also shows the location of the absorption energy levels assigned to the π-π*, n-π*, and surface-state transitions. The blue- and green-coded emission components are fairly constant in energy and most probably originate from absorption into the n-π* band and result in strong PL QYs. Under longer wavelength excitation there is less and eventually no n-π* excitation and emission results only from surface states with much lower efficiency. In the range of excitation energies studied herein, there was no PL observed associated with absorption into the π−π* band. The surface PL emission energy levels may arise from a distribution of different emissive trap sites on each CD surface,15−17,45 which is similar to other fluorescent carbon nanomaterials, for example, graphene quantum dots.46 The highest value of the PL QY (49%) at λex = 360 nm is associated with emission from levels that are associated with molecularlike features, that is, connected with particular functional groups (in this case mostly from the CO group). The tuning of the emission color with excitation wavelength is a simple consequence of the lowering of the excitation energy, leaving the various higher emission energy levels inaccessible (adding on the associated energy for the Stokes shift in each case). The n-π* and the broad surface-state absorption peaks overlap, and the emission energy levels are also fairly broad, numerous, and relatively evenly spaced, so there is no great interruption in the emission tuning and the color progression is quite smooth as the excitation wavelength is varied.

Figure 2. (a) PL spectrum of Si-CDs colloidal solution at 405 nm excitation. (b) PL decay curves of Si-CDs colloidal solution at 405 nm excitation detected at different emission wavelengths as indicated.

such a carbon dot ionogel (CD-ionogel) exhibits remarkable tunability of the emission color covering the full blue-to-red range of the visible spectrum. This useful property makes use of the simple effect of light reabsorption and subsequent emission by the Si-CD components of the ionogel. Optical Properties of Si-CDs. Si-CDs were synthesized by the pyrolysis of anhydrous citric acid in a coordinating solvent [3(2-aminoethylamino)propyl]trimethoxysilane (AEATMS) at 240 °C following the established protocol with a minor modification.22 Fourier transform infrared (FTIR) spectra confirm the formation of AEATMS-capped CDs by the presence of a characteristic peak at 1635 cm−1 assigned to the stretching vibration of CO groups from CONH units and two signals at 1034 and 1116 cm−1 from Si−O−C and Si− O−Si vibrations (Figure S2 in the Supporting Information and analysis22,39). Dynamic light scattering measurements indicated that the average particle size of the Si-CDs in dry ethanol was 1.2 nm (Figure S3 in the Supporting Information). As demonstrated in Figure 1c, colloidal solutions of Si-CDs show strong optical absorption in the UV region with three main features: two absorption bands centered at 280 and 356 nm, with narrow full widths at half-maximum (fwhm) of 44 and 58 nm, respectively, and a broad tail with a peak at 470 nm extending over the visible range (inset of Figure 1c). According to previous reports,40−42 the peak at 280 nm is due to a π−π* transition of the aromatic CC bond, while the peak at 356 nm is attributed to an n-π* transition of the CO bond, and the broad peak at 470 nm may originate from the multiaminosilane-functionalized surface of the Si-CDs. The excitation wavelength range of colloidal Si-CDs covers 260− 500 nm with a maximum efficiency at 360 nm. This can be seen as the red curve inset in Figure 1c, which shows the intensities of the PL emission peak of a series of PL emission spectra plotted for the range of corresponding excitation wavelengths (as shown on the wavelength abscissa). In other words, because the position of the emission peak is strongly excitation

Table 1. Fitting Parameters of the Corresponding PL Decay Curvesa λex [nm]

λem [nm]

B1 [%]

τ1 [ns]

B2 [%]

τ2 [ns]

τavg [ns]

χ2

405

417 457 497 537 577

87.1 83.5 78.6 75.6 79.8

10.1 10.0 10.1 10.5 10.8

12.9 16.5 21.4 24.4 20.2

3.1 2.5 3.1 3.7 3.8

9.8 9.7 9.6 9.8 10.2

1.07 1.13 1.20 1.13 1.05

See Supporting Information for definitions of fitting parameters. χ2 is the value of the goodness-of-fit parameter. B and τ correspond to the normalized amplitude and decay time constant. The systematic error is approximately ±0.5 ns for all lifetime values. a

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Figure 3. (a) Schematic illustration of the preparation of the CD-ionogel using Si-CDs through sol−gel processing in the presence of IL [CarbBim]Br. (b) Photographs of the free-standing pellets of CD-ionogel, which are both transparent and flexible. (c) UV/vis transmittance spectrum of the CD-ionogel with the thickness of 2.0 mm. (d,e) Storage (G′) and loss (G″) moduli measured on a frequency sweep in a range of ω = 0.01−100 rad s−1 at a fixed strain (γ = 0.25%) (d) and on a strain sweep in a range of γ = 0.01−1000% at a fixed frequency (ω = 1.0 rad s−1) (e).

A further small but noticeable contribution to the smoothing out of the tuning process can also be observed in the emission spectra where the excitation is close to the highest PL emission level, especially where the latter are associated with surface-state emission. In Figure S5 in the Supporting Information, we highlight several such cases where noticeable phonon-assisted excitation broadens the emission peak a little, slightly above the excitation energy. The representative PL decay curves of the colloidal Si-CDs probed at different emission wavelengths (Figure 2a) with excitation at 405 nm are shown in Figure 2b, with decay fitting results listed in Table 1. The average radiative lifetimes remain in the range of 9.6 to 10.2 ns and are almost emission wavelength independent. This indicates that even with the surface-state emission lifetimes do not change very markedly relative to emission from levels associated with the n-π* transitions. However, we will show later that this is not the case when multiple consecutive reabsorption and re-emission processes occur in a CD-ionogel. Preparation of Flexible and Transparent CD-Ionogels. The CDionogel was prepared by mixing of 45 wt % of Si-CDs acting as

a silica precursor with 55 wt % of carboxyl-functionalized IL [Carb-Bim]Br. As illustrated in Figure 3a, hydrolysis and condensation reactions of the AEATMS capping of the Si-CDs occurs in water and leads to the formation of an inorganic silica framework with the amino groups of AEATMS acting as basic catalysts for the sol−gel process.39 Gelation of the ionogel occurred readily at 60 °C within 3.5 h and was then followed by aging for 48 h at room temperature and then 1 week at 60 °C to evaporate all volatile compounds, resulting in free-standing monolithic pellets of CD-ionogel, which were both highly flexible and transparent (Figure 3b). The FTIR spectrum of the CD-ionogel (Figure S2 in the Supporting Information) displays a strong and broad band at 1130 cm−1 ascribed to the Si−O−Si vibration, together with a weak band of Si−OH at 937 cm−1, which verifies the condensation of AEATMS during the sol−gel process and supports the formation of a 3-D inorganic network.38,39,47 The absorption bands at 1727 and 1163 cm−1 observed in the spectrum of [Carb-Bim]Br (Figure S2 in the Supporting Information) can be assigned to vibrations of the CO and C−O stretch modes for the −COOH group of the IL.48 These bands almost disappear, and new bands at 1566 and 1415

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Figure 4. (a) True-color photographs of five pellets of CD-ionogels with different thicknesses (indicated), all excited at 405 nm in the transmission geometry where excitation and detection points are on the opposite sides of the samples. Full-color tunability over the whole visible spectral range has been achieved by varying the pellet thickness from 0.1 to 10.0 mm. (b) Emission from CD-ionogel in an excitation-collection separated geometry with a range of distances between excitation and detection points. On the right, the experimental geometry showing laser excitation at a single wavelength (λ = 405 nm) with a sliding aperture for a precise control of the illumination-emission distance is shown; the PL signal is collected from the small edge of the sample. (c) Corresponding CIE coordinates of the CD-ionogel edge emission for a range of the distances between the excitation and collection points as presented in panel b. (d) PL decays of the CD-ionogel monitored at wavelengths corresponding to peaks of emission spectra in panel b (λex = 405 nm). (e) Emission energies corresponding to Gaussian peaks fitted to PL data for spatially separated excitation and emission as a function of emission propagation length in the CD-ionogel sample. The top panel shows the relative amplitudes of the peaks in the set at each distance. The amplitude bars are color-coded to match the emission energy curve points directly below each group. 1416

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Table 2. Fitting Parameters of the Corresponding PL Decay Curvesa λex [nm]

distance [mm]

λem [nm]

Ad1 [%]

τd1 [ns]

Ad2 [%]

τd2 [ns]

405

0.0 0.5 1.0 3.0 6.0 10.0 15.0

465 500 548 576 607 632 645

59.7 60.8 81.9 65.1 63.5 63.1 63.0

10.2 12.1 12.1 12.4 12.4 12.5 12.5

40.3 39.2

4.5 7.1

Ar [%]

18.1 34.9 36.5 36.9 37.0

τr [ns]

χ2

2.0 3.6 3.8 3.8 3.9

1.08 1.04 0.93 1.09 1.34 1.11 1.11

a See Supporting Information for definitions of fitting parameters. χ2 is the goodness-of-fit parameter. Ad and τd correspond to the normalized amplitude and decay time constant, and Ar and τr correspond to the normalized amplitude and rise time constant. The systematic error is approximately ±0.5 ns for all lifetime values.

The mechanical properties of ionogels are considered one of the most important factors for their practical and commercial applications. We investigated the rheological properties of the CD-ionogel. In Figure 3d, the storage moduli (G′) and loss moduli (G″) are shown as functions of angular frequency (ω) at a fixed strain (γ = 0.25%). Before the measured angular frequency at 50.0 rad s−1, the CD-ionogel showed a plateau region in G′ with its value always larger than G″, indicating that the elastic component of the dynamic modulus was greater and thus the CD-ionogel acted like a quasi-solid. The G″ values increased with angular frequency and exceed the G′ values above ω = 50.0 rad s−1, indicating the occurrence of a transition from solid-like to liquid-like behaviors (a gel−sol transition) for the CD-ionogel sample. The strain amplitude sweeps of the sample also demonstrated the elastic response of CD-ionogel (Figure 3e) in the lower strain range. These properties of the CD-ionogel are potentially useful for easy processability. Thickness-Dependent Full-Color Emission Tunability in a CDIonogel under Single Excitation Wavelength. Excitation-dependent emission is a commonly known property of CDs. We employed this marked optical property of CDs, which, when combined with high (45 wt %) concentrations of CDs dispersed in the ionogel matrix, resulted in consecutive reabsorption and reemission at progressively longer wavelengths as the emission propagation distance in the CD-ionogel was increased. This provides simple full-color tunability of the resulting emitted light. Figure 4a shows five pellets of CD-ionogels with different thickness, with the emission detected along the same axis as the illumination, under the same excitation wavelength of 405 nm in each case. As the thickness of the pellet increased from 0.1 to 10.0 mm, the PL emission was tuned from blue to red, over the whole visible spectral range. To further explore the mechanism of the color tuning and exclude any possible effects of varying concentration and inhomogeneity of different samples, we made a further set of measurements on a single stripe sample of the CD-ionogel with a size of 2.5 cm × 1.0 cm and a thickness of 0.2 cm using the geometry with masked edges shown in

Figure 5. (a) PL spectrum of CD-ionogel at 405 nm excitation (single spot excitation with emission collected from the same location). (b) Normalized PL decay curves of CD-ionogel at 405 nm excitation detected at different wavelengths, as indicated.

1452 cm−1 are observed in the FTIR spectrum of the CDionogel (Figure S2 in the Supporting Information), which can be assigned to the asymmetric and the symmetric stretching modes of the carboxylate group, respectively, suggesting the deprotonation of the carboxylic acid of the IL,49 as schematically shown in Figure 3a and Figure S6 in the Supporting Information. The energy-dispersive X-ray spectrum (EDS) of the CD-ionogel indicates the presence of C, N, O, Br, and Si as the primary chemical components with atomic ratios of 59.11, 11.53, 17.92, 5.71, and 5.73%, respectively (Figure S7 in the Supporting Information). The transmission spectrum of the CD-ionogel with a thickness of 2.0 mm shows that it is 90% transparent for wavelengths longer than 640 nm and at the same time absorbs all light with wavelengths shorter than 420 nm due to the high (45 wt %) concentration of the embedded Si-CDs (Figure 3c), resulting in an intense red color (Figure 3b). This very useful property of the CD-ionogel plays a crucial role for the achievement of the full-color tunability of its emission wavelength, as we discuss in detail later.

Table 3. Fitting Parameters of the Corresponding PL Decay Curvesa λex [nm]

λem [nm]

Bd1 [%]

τd1 [ns]

Bd2 [%]

τd2 [ns]

405

425 465 505 625 705

60.1 59.7 47.6 90.3 49.4

9.2 10.2 12.2 11.3 11.8

39.9 40.3 52.4

3.8 4.5 7.0

50.6

8.0

Br [%]

9.7

τr [ns]

τavg [ns]

χ2

8.1 8.9 10.1

1.15 1.08 1.06 1.23 1.32

1.5 10.2

a See Supporting Information for definitions of fitting parameters. χ2 is the goodness-of-fit parameter. Bd and τd correspond to the normalized amplitude and decay time constant, and Br and τr correspond to the normalized amplitude and rise time constant. The systematic error is approximately ±0.5 ns for all lifetime values.

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fitted to two-exponential decay fits and are depicted as solid curves in Figure 4d with the corresponding fit parameters summarized in Table 2. One remarkable observation is the relatively large delay of the PL signal maximum and the appearance of a progressively slower rise time component with increasing distance between excitation and detection. At virtually zero distance, the observed peak delay and rise time are instrument-limited and ≤0.1 ns, whereas after propagation through 15.0 mm of material the delay reaches 4.2 ns and the transient shows a rise time of 3.9 ns. Although the cumulative reabsorption and emission processes eventually shift the principle transitions involved from the more efficient n-π* carbonyl band down to the less efficient surface state transitions after several absorption-emission cycles, Figure 4d shows that (though less efficient) without any reabsorption the surfacestate recombination rates are not very different from those for higher energy excitations, with average decay lifetimes of ∼12.0 ns. In Figure 4d, the decay rates are again similar, and the rates in the tails of the decays are broadly similar. At the same time, the longer path length measurements show a progressively longer rise time and peak delay, which is not so evident in the short path length surface-state PL decays. It is therefore likely that the rise time and delay are amassed as a consequence of a number of absorption and emission cycles, with each emission delayed by a spread of times due to the stochastic nature of the PL process. At longer path lengths, after many such cycles of absorption and emission, the start of the decay transient becomes more rounded and the peak is delayed relative to the short path length case. The propagation-dependent red shift can even be observed in relatively thick (few millimeters) highly loaded gel samples in a top-face excitation geometry, where the excitation (front surface) and emission (detected also via the front surface) are at the same point on the sample. The PL spectrum shows an additional red-shifted shoulder at ∼625 nm (Figure 5a) due to significant reabsorption and emission. This is similar to the redshift mechanism observed previously in semiconductor quantum dots, for example, for CdSe/ZnS and PbS nanocrystals.50 In those cases, the red shift was explained by a process in which the emission from smaller dots was consecutively absorbed and re-emitted by larger ones. For the CD-ionogels containing high Si-CD concentrations, this process is likewise driven by consecutive reabsorptions, where each Stokes shift from the previous emission progressively reduces the highest accessible energy level for the next absorption. However, a wide span of energy levels arising from the n-π* and surface-state transitions allows this progressive down shift to continue smoothly over a wide energy range leading to a strong red shift. As with the spatially separated excitation and emission geometry, this can also be observed via time-resolved PL spectroscopy. By monitoring the PL decay profile at different emission wavelengths (as shown in Figure 5b) in contrast with the PL decay profile monitored at the main PL peak (λem = 465 nm), the PL decay on the red-shifted shoulder (λem = 625 nm) can only be fitted with an additional rise time component of 1.5 ns (the fitting parameters of PL decay curves, monitored at different emission wavelength, are listed in Table 3), indicating a slight delay of the PL signal peak due to consecutive reabsorption even with the relatively short emission propagation distances in the sample. In summary, a flexible, free-standing and transparent CDionogel was successfully fabricated by a facile sol−gel method. Through the simple variation of the thickness of the CD-

Figure 4b, which ensures that any fluorescence detected originates primarily from excitation at locations other than the output face of the sample. Single wavelength excitation was provided by a 405 nm diode laser (with a laser beam size of 0.5 mm × 2.0 mm) using a mask with a sliding aperture. Figure 4b shows that gradual tuning of the emission color from blue to red can be achieved by changing the distance between the excitation and the detection points. The corresponding CIE coordinates of the emitted light are shown in Figure 4c and listed in Table S1 in the Supporting Information, demonstrating the remarkable color purity for the red, green, and blue (primary RGB colors). Again, decomposing the broad complex emission peak into a number of subsidiary simple Gaussian peaks helps to reveal the mechanism behind the marked red shift in the emission color. In the same way as for Figure 1e, spectra were fitted to a sum of three Gaussian peaks for each illumination position, and the energies corresponding to the peak positions versus the illumination distance are shown in Figure 4e. As the emission propagates through progressively longer distances, the emission is reabsorbed and emitted an increasing number of times, and on each cycle the emission process lowers the photon energy by an amount equivalent to the Stokes shift for the transitions involved. The relative amount of light emitted from the highest level falls progressively until after several cycles there are an insufficient number of photons from the previous cycle with sufficiently high enough energies to populate the next highest level. As this is happening, the next lowest emission level grows in relative strength as the highest level fades, taking over as the highest level in turn. In Figure 4e, this succession of the highest emitting level can be seen to occur several times over as the length is extended to 15.0 mm. The marked feature of this red shift in Si-CDs is that the series of levels spans a wide range of energies (well over 1 eV), and because the emission and absorption levels are broad, the emission color tunes smoothly over this wide range (as seen in the CIE curve and photographs in Figure 4c,a, respectively). The length-dependent red shift can easily be modeled, even in a relatively approximate fashion, using the excitation-dependent PL data and sample transmission spectra such as in Figures 1d and 3c and simply calculating the transmitted and emission spectra for a small incremental sample length and taking the sum of these spectra as the excitation for the next length increment. Even when approximating the emitted spectrum from each length section as one (or for shorter path lengths two) single wavelength source for the next section, reasonable agreement in the location of the emission peak with those from experimental measurements can be obtained over several millimeter path lengths (Figure S8a in the Supporting Information), although this simple approach slightly underestimates the contribution from lower lying states for shorter path lengths. (That is, approximating the incremental excitation as a single wavelength source does not include all of the emission driven by longer wavelength excitations.) At longer path lengths, where the red emission is weaker, the changes in the fluorescence spectrum are dominated by the transmission spectrum of the gel (Figure 3c). Figure S8b in the Supporting Information shows the relative PL intensity of the CD-ionogel for a range of emission propagation lengths. To gain a better understanding of the recombination dynamics within the CD-ionogel with spatially separated excitation and detection points, we used time-resolved PL spectroscopy (Figure 4d). Transient PL−intensity profiles were 1418

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ionogel, we achieved full-color tunability of the fluorescence emission color with a fixed single excitation wavelength and a smooth spectral progression of emission colors covering the whole of the visible spectrum. This very marked red-shift effect is based on the combination of light reabsorption and the excitation-dependent PL of Si-CDs dispersed with high concentration in an ionogel. The CD-ionogels introduced here offer the combined advantages of PL color tuning, transparency, and flexibility. With further development, that is, in most cases not relying on reabsorption in thick films to address different emission colors but using other (e.g., electrooptic) tuning mechanisms, this work may lead to materials suitable for the development of flexible electroluminescent devices, solid-state lighting, color displays, and luminescent solar concentrators.

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ASSOCIATED CONTENT

S Supporting Information *

Details of experimental methods, characterization, PL spectra and TRPL fitting procedure, and additional data on characterization of ionogels. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

∥ Yu Wang and Sergii Kalytchuk contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Research Grant Council of Hong Kong S.A.R. (project no. [T23-713/11]) and the Ability R&D Energy Research Centre (AERC) of City University of Hong Kong. We thank Li Yan for the helpful suggestions on the preparation of the sample mold.

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