Defect-mediated spontaneous emission enhancement of plasmon-coupled CuInS<sub>2</sub> and CuInS<sub>2</sub>/ZnS

Quinton Rice; Sangram Raut; Rahul Chib; Anderson Hayes; Zygmunt Gryczynski; Ignacy Gryczynski; Young-Kuk Kim; Bagher Tabibi; Jaetae Seo

doi:10.1364/OME.6.000566

1. Introduction

The hybrid optical materials of metal nanoparticles (MNPs) and semiconductor quantum dots (QDs) have been of great interest for photonic applications [1–18]. The strong plasmon-exciton coupling in the hybrid optical materials of MNPs and QDs through the Coulomb interaction gives rise to the high quantum efficiency because of the large spontaneous emission enhancement and strong local field [8]. The II-VI semiconductor QDs provide high color purity with wide optical tunability and strong blue-shift by the quantum confinement of excitons within the dot boundary with sizes near the bulk exciton Bohr radius [8]. The I-III-VI₂ semiconductor QDs have wide spectral distribution and are environmentally friendly compared to the semiconductor compounds that include the element of cadmium, lead, selenium, or tellurium [19, 20]. The merits of I-III-VI₂ QDs have led to the strong attention on the copper-indium-disulfide (CIS) QDs for developing white light generation through the spectral coupling with excitation source [21]. It is known that the wide spectral distribution of CIS is mainly due to the transitions from surface/interface-, shallow-, and deep-trapped states by the defects of chalcopyrite, whereas the narrow spectral width of II-VI QDs is mainly due to the bandedge transition in zincblende or wurtzite structure [8, 19]. The chalcopyrite crystal structure of I-III-VI₂ ternary compound is formed by the regular substitutions of I-III group atoms for the II-group lattice in the zincblende structure of II-VI binary compound [22, 23]. The surface-trapped state of CIS QDs arises from the discontinuity of crystal regularity, dangling atoms of I-III or chalcogenide, and/or any impurities on the surface of nanocrystals. The interface-trapped state of CIS/ZnS coreshells is occurred by the smaller lattice parameters of shell materials than those of core materials. The shallow- and deep-trapped states are formed by the compositional vacancies and/or rich substitutions for the vacancies [19, 24–26] in the chalcopyrite crystal structure as shown in Fig. 1(a). The shallow defect-related state is due to the sulfur vacancy (V_s) below the conduction band and the copper-rich (Cu_In) above the valence band. The deep defect-related state is due to the indium interstitial (In_i) or indium-rich as indium atom in the copper vacancy (In_Cu) below both the conduction band and the shallow-trapped state as a donor (D) and above both the valence band and shallow-trapped state as an acceptor (A). The defect-related CIS with and without ZnS QD ensembles conforms the energy levels as the density of states of surface/interface-, shallow-, and deep-trapped exciton pairs due to the irregularities and defects of the QDs.

Fig. 1 (a) Chalcopyrite crystal structure of I-III-VI₂ ternary compound, and its defect structures of CuInS₂ for the shallow- and deep-trapped states; (b) Schematic diagrams of plasmon-exciton coupling and energy transitions and interactions, where ST/IT is the surface trapped/interface trapped states, D_s is the shallow donor state, D_D is the deep donor state, A_S is the shallow acceptor state, and A_D is the deep acceptor state; and (c) Bulk stacks of CIS or CIS/ZnS QDs and Au MNPs ensembles on a micro-glass plate.

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When the CIS QDs are placed in the vicinity of plasmonic MNPs, the defect-trapped excitons are strongly coupled to the plasmons through the Coulomb interaction. Responding to the external excitation, the interband energy transitions of excitons in QDs and the intraband energy transitions of plasmons in MNPs occur and intermingle with each other through the Coulomb interactions. The schematic diagram of energy transitions and transfers through plasmon-exciton coupling in hybrid optical material of QDs and MNPs is illustrated in the Fig. 1(b). The energy transfer includes the direct energy transfer between QDs and MNPs, and stepwise transfer between QD1, QD2, and MNPs. The hybrid collective plasmon-exciton excitations and transitions lead to the plasmon-enhanced exciton emission by the strong plasmon-exciton coupling and intense local field.

The Purcell enhancement factor (F_p) [27] is quantified by the ratio of the PL decay rate of hybrid QD-MNPs to that of QDs without MNPs, which indicates the enhancement of spontaneous emission decay-rate in the hybrid QD-MNPs. If the coupling time of plasmon-exciton is much faster than the radiative and nonradiative lifetimes of excitons, the total PL decay rate of hybrid QD-MNPs is much larger than the PL decay rate of QDs without MNPs that indicates large spontaneous emission enhancement. This implies that, for the large Purcell enhancement factor, even dark emission materials can achieve the high quantum efficiency. Therefore, the plasmon-exciton coupling leads to the realization of nanophotonic chips or devices with the high quantum efficiency and fast light emission.

The spontaneous emission from plasmon-coupled semiconductor QDs have been extensively reported, however the PL enhancement of defect-related chalcopyrite QDs with plasmon coupling is not reported yet. The defect-mediated spontaneous emission enhancement of plasmon-coupled CIS and CIS/ZnS QDs is the first report within our best knowledge. The hybrid optical materials consist of bulk stacks of CIS QDs and Au MNPs ensembles on the micro-glass quartz plate as shown in Fig. 1(c). The fractional defect-mediated spontaneous emission enhancement of plasmon-coupled CIS and CIS/ZnS QDs is studied for the white light-emitting-diode (LED) applications. The time-resolved PL studies with three-exponential decays at lower, intermediate, and longer spectral regions illustrate the fractional amplitude and spontaneous emission enhancement of plasmon-coupled defect-related excitons of CIS and CIS/ZnS at the surface- or interface-related, shallow- and deep-trapped states. The temperature-dependent PL studies of CIS and CIS/ZnS QDs with and without plasmonic coupling reveal the temperature-sensitive and fractional defect-mediated PL enhancement. Finally the PL enhancement based on the defect-mediated spontaneous emission enhancement of CIS and CIS/ZnS QDs with plasmon coupling compared to the PL of QDs without plasmon coupling is elucidated.

2. Experimental details

The material preparations of CIS and CIS/ZnS QDs and Au MNPs were described elsewhere [19, 28–33]. The absorption spectra of CIS and CIS/ZnS QDs and Au MNPs were measured with an UV-VIS spectrophotometer (Cary 50 Bio, Varian Inc.) and an UV-VIS spectrometer (Agilent 8453).

The time-resolved defect-mediated PL decays of CIS and CIS/ZnS with and without Au nanoparticles were measured using a FluoTime 200 fluorometer (PicoQuant, Inc.) with a diode laser excitation at ~470 nm with a pulse width less than ~120 ps and 100 KHz repetition rate. The instrument response function (IRF) for the PL decay measurement was ~512 ps maximum. The fluorometer was equipped with a microchannel plate detector (MCP, Hamamatsu, Inc). Two long wavelength pass (LWP) filters at ~495 nm and ~520 nm were placed before the detector to exclude laser irradiation into the PL decay measurement. The PL decay measurement wavelengths were 600 nm, 650 nm, and 710 nm for CIS with/without Au MNPs and 535 nm, 595 nm, 650 nm for CIS/ZnS with/without Au MNPs to study the defect-mediate PL decays due to the surface-/interface-trapped, shallow defect-related, and deep defect-related states at shorter, peak, and longer spectral regions. The spectral bandwidth of PL decay measurements was ~1 nm. The PL lifetimes of QDs with/without plasmonic MNPs were estimated by a tail fitting with the multi-exponential decay function and the nonlinear least square function to the decay measurements, which were described by Seo et al [19], using the FluoFit4 program (PicoQuant, Inc.).

The temperature-dependent PLs from CIS and CIS/ZnS QDs with/without Au MNPs were detected by a spectrometer (Ocean Optics, USB4000) with a spectral resolution of ~1 nm though a multi-mode optical fiber (Ocean Optics, P600-VIS-NIR). The excitation source was a HeCd laser at 325 nm in CW mode with laser power of ~9 mW through an optical chopper operating at a frequency of 300 Hz. The CIS and CIS/ZnS QDs with/without Au MNPs were placed between two quartz micro-glasses on the cold-finger in vacuum chamber of helium closed-cycle cryostat (Janis, SHI-4-1) which is outfitted with a cooling He-compressor unit (Sumitomo, CNA-11 C). The temperature of CIS and CIS/ZnS QDs with/without Au MNPs was controlled by a thermal diode with temperature controller (Lakeshore 331) from ~6 K to ~300 K. The excitation power was monitored for the PL measurement at each temperature, and the time interval for temperature stabilization after each temperature change of 10K was over 10 minutes.

3. Results and discussion

The absorption spectra of CIS QDs, CIS/ZnS QDs, and Au MNPs; the PL spectra of CIS with/without Au MNPs and CIS/ZnS with/without Au MNPs at room temperature, and the excitation laser spectrum are shown in Fig. 2(a) for CIS QDs and (b) CIS/ZnS QDs. The absorption spectra of CIS and CIS/ZnS show a strong blue-shift from the bandgap (~1.45 eV / ~855 nm) of bulk materials [20]. The strong blue-shift is due to the quantum confinement of excitons within the dot boundary with the sizes near the bulk Bohr radius of ~4.1 nm. However, the absorption spectra does not exhibit the distinct discrete absorption peaks and sharp bandedge, but displayed a long tail due to the wide spectral distribution of defect-related surface- / interface-, shallow-, and deep-trapped states, and the typical wide size-distribution from the colloidal nanoparticles synthesis [19]. The Au MNP absorption spectra show the interband transition near UV region, and the intraband spectra around the 532 nm which is called the localized surface plasmon resonance (LSPR) as shown in Fig. 2 [33]. The pinkish visual appearance of Au nanoparticles is due to the residual transmittance of white light after the photons at blue and green spectra are absorbed by the colloidal nanoparticles.

Fig. 2 Absorption and PL spectra of (a) CIS and (b) CIS/ZnS, absorption spectrum of plasmonic Au MNPs, and laser excitation spectrum.

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The PL peaks of CIS with/without Au MNPs were ~660 nm (FWHM: ~105 nm) and ~685 nm (FWHM: ~110 nm), and the PL peaks of CIS/ZnS with/without Au MNPs were ~615 nm (FWHM: ~118 nm) and ~609 nm (FWHM: ~149 nm). These spectroscopic parameters indicate that the exciton energies for the PL transitions in CIS/ZnS QDs are closer to the LSPR than those for the PL transitions in CIS QDs. The close resonant plasmon-exciton coupling may result in the larger PL enhancement. In comparison to the PL peaks of CIS and CIS/ZnS, the PL peaks of plasmon-coupled CIS and CIS/ZnS displayed ~25-nm blue-shift and ~6-nm red-shift, respectively. The blue-shift PLs from plasmon-coupled QDs may imply the higher fractional contributions of surface-trapped and shallow-trapped states at shorter spectral region to the PL from the plasmon-coupled CIS compared to the fractional contributions of defect-related states to the PL from the bare CIS. The red-shifted PLs from plasmon-coupled QDs may imply the higher fractional contributions of surface-trapped state at longer spectral region or deep-trapped state to the PL from the plasmon-coupled CIS/ZnS compared to the fractional contributions of defect-related states to the PL from the bare CIS/ZnS. The time-resolved PL studies may provide the better understanding on the defect-mediated PL enhancement.

Rice [21], Seo [19], Komarala [24], Nose [34], Hamanaka [35], Zhong [36], Unold [37], Castro [24] and their coauthors’ described that the fast decay time $(τ_{1})$ , intermediate decay time $(τ_{2})$ , and slow decay time $(τ_{3})$ are attributable to the surface- or interface-trapped state, and defect-related shallow- and deep-trapped states, respectively. According to the hypothetical drawing of energy diagram [19, 24–26] shown in Fig. 1(b), the surface- or interface-, defect-related shallow-, and defect-related deep-trapped states can be assigned to the shorter spectral region or higher energy, peak spectral region or intermediate energy, and longer spectral regions or lower energy, respectively. As Ueng and Hwang reported, the multi-energy levels with intrinsic defects can significantly influence each other, and the PL decay times with fast, intermediate, slow components are distributed over the entire spectral regions. Therefore, the time-resolved PL decays at different spectral regions reveal the temporal properties and fractional contributions of surface- or interface-, shallow- and deep-trapped excitons to the QD PLs, and explain the defect-mediated spontaneous emission enhancement.

The time-resolved PLs of CIS QDs and plasmon-coupled CIS QDs are shown in Fig. 3(a), 3(b), and 3(c), and their PL decays in logarithm scale are shown in (d), (e), and (f) at the measurement wavelengths of 600 nm, 650 nm, and 710 nm with 1-nm spectral width. The PL decay properties of CIS QDs and plasmon-coupled CIS QDs are summarized in Table 1. The PL decay properties include the lifetimes with three-exponential decays, averaged lifetimes, and their fractional amplitudes at the three spectral regions and three decay times. The defect states include the surface-trapped state with the fastest lifetime component at the lowest spectral region, the shallow-trapped state with the intermediate lifetime component at the peak spectral region, and the deep-trapped state with the slowest lifetime component at the longer spectral region, although all defect-related states are widely spread over the entire spectral region. The fractional defect-mediated spontaneous emission enhancement of plasmon-coupled CIS QDs include the largest fractional amplitude at the fastest life component with gradual decrease at longer spectral region, the medium fractional amplitude at the intermediate lifetime component with gradual increase at longer spectral region, and the lowest fractional amplitude at the slowest lifetime component with the large increase of fractional amplitude at longer spectral region. The enhanced fractional amplitude at the fastest life component and the gradual decrease at longer spectral region and at slower decay time components of plasmon-coupled CIS compared to those of CIS imply the blue-shift of PL of plasmon-coupled CIS.

Fig. 3 PL intensity decays, residuals, and exponential decay components of CIS and plasmon-coupled CIS on cover glass at (a) 600 nm, (b) 650 nm, and (c) 710 nm. PL intensity decay in logarithm scale at (d) 600 nm, (e) 650 nm, and (f) 710 nm

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Table 1. PL lifetimes of CIS and plasmon-coupled CIS with three-exponential decays, averaged lifetimes, and their fractional amplitudes at the three spectral regions

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The averaged lifetime of three-exponential decays are spectrally distributed ~11.3 ns, ~29.8 ns, and ~92.3 ns for uncoupled CIS, and ~5.5 ns, ~13.6 ns, and ~32.7 ns for plasmon-coupled CIS at the shorter, peak, and longer spectral region, respectively. The decay rates of defect-mediated carriers for the plasmon-coupled CIS QDs are ~2.1, ~2.2, and ~2.8 fold compared to those of CIS QDs at shorter, peak, and longer spectral region. The spontaneous emission enhancement of Au-CIS is attributable to the plasmon-exciton coupling at the surface-, shallow-, and deep-trapped states through Coulomb interaction and the local field enhancement.

The time-resolved PLs of CIS/ZnS QDs and plasmon-coupled CIS/ZnS QDs are shown in Fig. 4(a), 4(b), and 4(c), and their PL decays in logarithm scale are shown in (d), (e), and (f) at the measurement wavelengths of 535 nm, 595 nm, and 650 nm with 1-nm spectral width. The PL decay properties of CIS/ZnS QDs and plasmon-coupled CIS/ZnS QDs are summarized in Table 2. The PL decay properties of CIS/ZnS and plasmon-coupled CIS/ZnS include the PL decay times with three-exponential fitting, averaged lifetimes, and their fractional amplitudes at the lower, peak, and longer spectral regions.

Fig. 4 PL intensity decays, residuals, and exponential decay components of CIS/ZnS and plasmon-coupled CIS/ZnS on cover glass at (a) 535 nm, (b) 595 nm, and (c) 650 nm. PL intensity decays in logarithm scale at (d) 535 nm, (e) 595 nm, and (f) 650 nm.

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Table 2. PL lifetimes of CIS/ZnS and plasmon-coupled CIS/ZnS with three-exponential decays, averaged lifetimes, and their fractional amplitudes at the three spectral regions

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The defect states of CIS/ZnS QDs include the interface-trapped state with the fastest lifetime component at the lowest spectral region, the shallow-trapped state with the intermediate lifetime component at the peak spectral region, and the deep-trapped state with the slowest lifetime component at the longer spectral region, although all defect-related states are widely spread over the entire spectral region. The fractional defect-mediated spontaneous emission enhancement of plasmon-coupled CIS/ZnS QDs include the largest fractional amplitude at the fastest life component with gradual increase at longer spectral region, the medium fractional amplitude at the intermediate lifetime component with gradual decrease at longer spectral region, and the lowest fraction amplitude at the slowest lifetime component with slight increase at longer spectral region. The fractional amplitudes of shortest lifetime for the interface-trapped state are ~78%, ~84%, and ~86% at the shorter, peak, and longer spectral region, which indicate the large fractional contribution of interface-trapped state to the PL enhancement. The large fractional amplitude and gradual increase at longer wavelength for the fastest decay time component and the gradual increase in the longer wavelength for the slowest decay time component imply the red-shift of PL of plasmon-coupled CIS/ZnS.

The averaged lifetime of three-exponential decays for CIS/ZnS are spectrally distributed ~31.3 ns, ~65.5 ns, and ~64.7 ns for uncoupled CIS/ZnS, and ~1.3, ~2.0 ns, and ~2.6 ns for plasmon-coupled CIS/ZnS at the shorter, center, and longer spectral region, respectively. The decay rates of carriers of plasmon-coupled CIS/ZnS QDs are ~24.1, ~32.8, and ~24.9-fold at shorter, peak, and longer spectral region. The larger spontaneous emission enhancement with CIS/ZnS compared to CIS is due to the relatively stable carriers at the interface-trapped state and the relatively unstable carriers at the surface-trapped state of the QDs on the glass substrate, and the closer PL transitions of CIS/ZnS to the LSPR compared to that of CIS.

The temperature-dependent PL spectroscopy provides the deeper understanding on the fractional contribution of defect-mediate PL enhancement and spectral distribution, and the PL thermal quenching of the hybrid optical materials of Au-CIS and Au-CIS/ZnS through plasmon-exciton coupling.

The temperature-dependent PLs of CIS, plasmon-coupled CIS, CIS/ZnS, and plasmon-coupled CIS/ZnS are shown in Fig. 5 and 6 respectively. The red dash lines mark around the PL peaks at different temperatures from 6 K to 300 K, and the blue dash line indicates the PL peak changes as the temperatures changes. The large spectral blue-shift (~25 nm at 300 K and ~17 nm at 6 K) of PL peaks of plasmon-coupled CIS (~660 nm) in comparison with the PL peaks of CIS (~685 nm at 300 K and ~677 nm at 6 K) is attributable to the defect-mediated PL enhancement at the surface- and shallow-trapped states. It can be hypothesized that the temperature-dependent PL peak changes of CIS are due to the bandgap changes or surface-trapped state changes. However, there is no explicit PL-peak changes as the temperature changes for CIS/ZnS which is shown in Fig. 6. It implies that the temperature-dependent PL-peak changes of CIS are mainly due to the surface-trapped state changes. The spectral red-shift (~6 nm) of PL peaks for the plasmon-coupled CIS/ZnS in comparison with the PL peaks for CIS/ZnS is accredited to the defect-mediated PL enhancement by the higher fractional amplitude at the interface-trapped state around the longer spectral region.

Fig. 5 Temperature-dependent PL spectra of CIS and Au-CIS. The red dash line marks around the PL peaks, and the blue dash line indicates the PL peak changes as the temperature changes. Inset: Integrated PL intensity in logarithm scale as a function of the inverse temperature.

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Fig. 6 Temperature-dependent PL spectra of CIS/ZnS and Au-CIS/ZnS. The red dash line marks around the PL peaks. Inset: Integrated PL intensity in logarithm scale as a function of the inverse temperature.

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The insets in Fig. 5 and 6 display the integrated PL intensity in logarithmic scale as a function of the inverse temperature for CIS, plasmon-coupled CIS, CIS/ZnS, and plasmon-coupled CIS/ZnS. The fitting is based on the PL thermal quenching equation [19, 38]:

I_{P L} ~ \frac{1}{1 + T^{3 / 2} \sum_{i} C_{i} \exp (- E_{a c t, i} / k_{B} T)}

for multi-level systems with the approximations of fractional amplitudes and thermal activation energies. The fractional amplitudes (C_i) of activation ionization energy (E_act,i) are described by the fitting parameters

E_{1} \leq 1 m e V

with

C_{1} ~ 3 \times 10^{- 4}

,

E_{2} ~ 10 m e V

with

C_{2} ~ 9 \times 10^{- 7}

, and

E_{3} ~ 30 m e V

with

C_{3} ~ 1 \times 10^{- 8}

for CIS;

E_{1} ~ 2 m e V

with

C_{1} ~ 2 \times 10^{- 4}

,

E_{2} ~ 15 m e V

with

C_{2} ~ 1 \times 10^{- 4}

, and

E_{3} ~ 50 m e V

with

C_{3} ~ 9 \times 10^{- 4}

for plasmon-coupled CIS;

E_{1} \leq 1 m e V

with

C_{1} ~ 4 \times 10^{- 4}

,

E_{2} ~ 50 m e V

with

C_{2} ~ 5 \times 10^{- 2}

, and

E_{3} ~ 70 m e V

with

C_{3} ~ 9 \times 10^{- 1}

for CIS/ZnS; and

E_{1} ~ 3 m e V

with

C_{1} ~ 4 \times 10^{- 4}

,

E_{2} ~ 70 m e V

with

C_{2} ~ 5 \times 10^{- 3}

, and

E_{3} ~ 90 m e V

with

C_{3} ~ 5 \times 10^{- 4}

for plasmon-coupled CIS/ZnS. The E₁ and E₂ are related to the shallow states of surface-/interface-trapped states and shallow-defect states, and E₃ is related to the deep-defect states. The charges at surface-/interface-trapped states and shallow-defect states are thermally active compared to the charges at deep-trapped states, and the PL lifetimes at surface-/interface-trapped states and shallow-defect states are relatively shorter than those at at deep-trapped states. The fitting sensitivity by the deep-defect state to the PL thermal quenching is extremely weak within the given temperature range. The fitting parameters indicate that the plasmon-exciton coupling reduces the PL thermal quenching. The reduction of non-radiative decays in addition to the strong local field leads to the PL enhancement.

The Fig. 5 and 6 insets also clearly show the PL enhancement of plasmon-coupled QDs compared to the PL from uncoupled QDs. The PL enhancements of plasmon-coupled CIS at room temperature and 6 K were two-fold and three-fold compared to the integrated CIS PLs which is shown in Fig. 5 inset. The PL enhancements of plasmon-coupled CIS/ZnS at room temperature and 6 K were five-fold and eight-fold compared to the integrated CIS/ZnS PLs as shown in Fig. 6 inset. The Purcell enhancement factors (F_p) at room temperature were estimated to be F_p ~2.1, ~2.2, and ~2.8 folds for plasmon-coupled CIS, and F_p ~24.1, ~32.8, and ~24.9 folds for plasmon-coupled CIS/ZnS at the shorter, peak, and longer spectral region respectively. It may imply that the PL efficiency of uncoupled CIS/ZnS (at F_p = 1 without coupling $γ_{p l - e x} = 0$ ) is higher than that of uncoupled CIS (at F = 1 without coupling $γ_{p l - e x} = 0$ ). Dark emission materials with relatively lower PL efficiency can achieve large PL enhancement even with small spontaneous emission enhancement (F_p<<10) through plasmon-exciton coupling, however bright emission materials require large spontaneous emission enhancement (F_p>>10) for large PL enhancement through plasmon-exciton coupling. The temperature-dependent PL studies reveals that plasmon-exciton coupling implies that the radiative decay rate and plasmon-exciton coupling rate are dominant at the lower temperatures while the non-radiative decay rate is heavily involved in the energy transitions at the higher temperatures. The larger PL enhancement of plasmon-coupled CIS/ZnS in comparison with that of plasmon-coupled CIS is accredited to the significant spontaneous emission enhancement for the bright emission materials.

4. Conclusion

The time-resolved PL studies of Au-CIS, CIS, Au-CIS/ZnS, and CIS/ZnS QDs revealed the defect-mediated spontaneous emission enhancement, where the defect states include surface-/interface-, shallow-, and deep-trapped states. The decay rates indicated ~2.1, ~2.2, and ~2.8 fold Purcell enhancement factors for the plasmon-coupled CIS, and ~24.1, ~32.8, and ~24.9 fold Purcell enhancement factors for the plasmon-coupled CIS/ZnS at shorter, center, and longer spectral regions. The temperature-dependent PL studies of Au-CIS, CIS, Au-CIS/ZnS, and CIS/ZnS QDs revealed the defect-mediated PL enhancement. The large spectral blue-shift of plasmon-coupled CIS in comparison with the PL peaks of CIS is attributable to the defect-mediated PL enhancement at the surface- and shallow-trapped states. The spectral red-shift of PL peaks for plasmon-coupled CIS/ZnS in comparison to the PL peaks for CIS/ZnS is accredited to the defect-mediated PL enhancement by the higher fractional amplitude at the interface-trapped state around the longer spectral region. The PL enhancements of plasmon-coupled CIS at room temperature and 6 K were two-fold and three-fold compared to the integrated CIS PLs, and the PL enhancements of plasmon-coupled CIS/ZnS at room temperature and 6 K were five-fold and eight-fold compared to the integrated CIS/ZnS PLs. This could be explained by the PL enhancement as a function of spontaneous emission enhancement of plasmon-coupled QDs for the dark and bright emission materials of uncoupled QDs. The temperature-dependent PL studies indicate that the PL enhancement with plasmon-exciton coupling can be achieved by suppressing the non-radiative decay. The larger PL enhancement of plasmon-coupled CIS/ZnS in comparison with that of plasmon-coupled CIS is accredited to the significant defect-mediated spontaneous emission enhancement for bright emission materials. The spontaneous emission enhancement of plasmon-coupled QDs is attributable to the plasmon-exciton coupling with fractional contribution of the defect-related carrier pairs through Coulomb interaction and the local field enhancement.

Acknowledgment

The work at HU was supported by the National Science Foundation (NSF) HRD 1137747 and W911NF-11-1-0177. The work at TCU and UNT Health Science Center was supported by the National Institutes of Health (NIH) grant R01EB12003 (Z.G) and National Science Foundation (NSF) grant CBET-1264608 (I.G).

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CIS	600 nm	650 nm	710 nm
τ₁	4.7 ns (61.0%)	6.8 ns (51.4%)	15.0 ns (24.9%)
τ₁ w/Au	1.7 ns (63.85%)	3.2 ns (57.5%)	5.5 ns (48.3%)
τ₂	19.1 ns (34.8%)	33.4 ns (38.6%)	59.8 ns (44.2%)
τ₂ w/Au	8.5 ns (32.0%)	17.2 ns (35.5%)	32.5 ns (39.1%)
τ₃	86.6 ns (4.3%)	134.0 ns (10.1%)	201.2 ns (30.9%)
τ₃ w/Au	49.9 ns (3.2%)	80.0 ns (7.0%)	137.1 ns (12.6%)
τ_ave	13.2 ns	29.8 ns	92.3 ns
τ_ave w/Au	5.5 ns	13.6 ns	32.7 ns

CIS/ZnS	535 nm	595 nm	650 nm
τ₁	5.9 ns (47.4%)	7.9 ns (37.9%)	3.5 ns (45.2%)
τ₁ w/Au	0.4 ns (78.4%)	0.6 ns (83.7%)	0.6 ns (85.5%)
τ₂	32.9 ns (41.4%)	54.3 ns (41.0%)	41.0 ns (26.3%)
τ₂ w/Au	2.5 ns (19.8%)	4.0 ns (14.5%)	4.6 ns (12.4%)
τ₃	132.3 ns (11.2%)	176.4 ns (21.2%)	183.1 ns (28.5%)
τ₃ w/Au	22.7 ns (1.8%)	53.2 ns (1.9%)	72.0 ns (2.1%)
τ_ave	31.3 ns	65.5 ns	64.7 ns
τ_{ave w/Au}	1.3 ns	2.0 ns	2.6ns

CIS	600 nm	650 nm	710 nm
τ₁	4.7 ns (61.0%)	6.8 ns (51.4%)	15.0 ns (24.9%)
τ₁ w/Au	1.7 ns (63.85%)	3.2 ns (57.5%)	5.5 ns (48.3%)
τ₂	19.1 ns (34.8%)	33.4 ns (38.6%)	59.8 ns (44.2%)
τ₂ w/Au	8.5 ns (32.0%)	17.2 ns (35.5%)	32.5 ns (39.1%)
τ₃	86.6 ns (4.3%)	134.0 ns (10.1%)	201.2 ns (30.9%)
τ₃ w/Au	49.9 ns (3.2%)	80.0 ns (7.0%)	137.1 ns (12.6%)
τ_ave	13.2 ns	29.8 ns	92.3 ns
τ_ave w/Au	5.5 ns	13.6 ns	32.7 ns

CIS/ZnS	535 nm	595 nm	650 nm
τ₁	5.9 ns (47.4%)	7.9 ns (37.9%)	3.5 ns (45.2%)
τ₁ w/Au	0.4 ns (78.4%)	0.6 ns (83.7%)	0.6 ns (85.5%)
τ₂	32.9 ns (41.4%)	54.3 ns (41.0%)	41.0 ns (26.3%)
τ₂ w/Au	2.5 ns (19.8%)	4.0 ns (14.5%)	4.6 ns (12.4%)
τ₃	132.3 ns (11.2%)	176.4 ns (21.2%)	183.1 ns (28.5%)
τ₃ w/Au	22.7 ns (1.8%)	53.2 ns (1.9%)	72.0 ns (2.1%)
τ_ave	31.3 ns	65.5 ns	64.7 ns
τ_{ave w/Au}	1.3 ns	2.0 ns	2.6ns

Defect-mediated spontaneous emission enhancement of plasmon-coupled CuInS₂ and CuInS₂/ZnS

Abstract

1. Introduction

2. Experimental details

3. Results and discussion

4. Conclusion

Acknowledgment

References and links

Cited By

Figures (6)

Tables (2)

Equations (1)

Optical Materials Express