Article pubs.acs.org/Langmuir
Size-Dependent Color Tuning of Efficiently Luminescent Germanium Nanoparticles Naoto Shirahata,*,†,‡,§ Daigo Hirakawa,‡,∥ Yoshitake Masuda,⊥ and Yoshio Sakka‡,∥ †
International Center for Materials Nanoarchitectonics (WPI-MANA), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan § Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan ∥ Graduate School of Pure and Science and Applied Science, The University of Tsukuba, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ⊥ National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama, Nagoya 463-8560, Japan ‡
S Supporting Information *
ABSTRACT: It is revealed that rigorous control of the size and surface of germanium nanoparticles allows fine color tuning of efficient fluorescence emission in the visible region. The spectral line widths of each emission were very narrow ( 5 ns) components becomes measurable. Such slow relaxation would be explained by the trapped carrier recombination in the ground state, leading to the appearance of long time-resolved emission tails (Figure S3, Supporting Information). Interestingly, relatively higher visible PL QYs have always been observed from the partially oxidized Ge nanoparticles unlike nonoxidized ones, suggesting a significant role of interfacially related e−h recombination for efficient visible light emission.
Table 2. Room-Temperature PL Properties of Five Different Ge Nanoparticle Fractionsa lifetime, τ(ns) sample
PL QY, ϕ(%)
PL fwhm (meV)
τ1B1 (%)
τ2B2(%)
χ2
0.86 68 0.92 85 1.52 82 1.51 79 2.06 64
3.10 32 2.10 85 5.18 18 6.72 21 8.69 36
1.37
near-UV PL
8
480
violet PL
9
170
blue PL
15
450
light-blue PL
12
500
4
420
green PL a
1.02 1.06 1.05 1.57
The estimates contain a maximum uncertainty of 0.2%.
the PL QYs of Ge nanoparticles. The best PL QY performance reported to date is in the blue spectral regions, ϕ = 11%.8 Other reported values tend to be in the range of ϕ ≤ 1%, which does not satisfy the requirements (ϕ > 1%) even for fluorescence applications such as package tagging and biomedical imaging. Significantly, our luminescent Ge nanoparticles show sufficiently high PL QYs (ϕ = 4%−15%) to qualify for labeling use. The poor optical performance of Ge as a light emitter, as reported in earlier studies, can be explained by time-resolved PL spectroscopy. PL intensity I(t) can be expressed as a function of time
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CONCLUSIONS We report here the superior light-emission properties of Ge nanoparticles. Bulk crystalline Ge shows poorer optical performance, and its indirect band gap character is believed to be inherited even in its nanostructures, resulting in emission spectrum broadness and low PL QYs. In the present study, we found that such a broad emission spectrum can be separated into narrow spectral lines by taking advantage of the small difference in surface polarity between nanoparticles and subsequent size purification. In addition to narrower fluorescence spectra, the nanoparticles obtained by emissioncolor separation emit efficiently in the near-UV, violet, blue, light-blue, and green spectral regions. The violet-light-emitting nanoparticle is a new family of luminescent Ge.
I(t ) = B1e−t / τ1 + B2 e−t / τ2
where B and τ are constants determined by curve-fitting decay profiles and are listed in Table 2. The estimated lifetime is expressed as the sum of lifetimes for radiative recombination (τr) and nonradiative recombination (τnr). Because the inverse lifetime is expressed as the recombination probability per unit time, the lifetime determined by both carrier transition processes is expressed as 1/τt = 1/τr + 1/τnr. Thus, PL QY is described by the equation ϕ = 1/(1 + (τr/τnr)). In general, the inefficient recombination process of photogenerated carriers is observed from bulk Ge. However, our Ge nanoparticles efficiently emit light in the visible region. Our findings reported herein reaffirm the significance of nanoparticle size as the prime structural parameter for tuning the light-emission wavelength. In fact, tuning requires control of the both particle size and the particle-size distribution, to a far greater extent than we had predicted. In comparing the optical absorbance and emission spectra in Figure 8, we notice that an additional structural parameter (i.e., surface configuration) is also important to performing color tuning of the emission with a high PL QY. Our obtained PLE spectra can be categorized by spectral shape into two groups: one that includes near-UV and violet luminescent fractions and one that includes all other luminescent fractions. In the spectra of the first group (nearUV and violet luminescent fractions), we see the PLE maxima at photon energies corresponding to structural features that appear as shoulders in the absorption spectra. In addition, PL relaxation processes are essentially dominated by fast components with lifetimes of τ < 1 ns. These emission features are very similar to the properties observed from the nonoxidized Si nanoparticles in which photogenerated carriers are confined for the appearance of the quantum size effect.32,35 In contrast, all of the spectra of the second group (all other
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ASSOCIATED CONTENT
S Supporting Information *
1
H NMR spectra of the 1-octene-treated Ge nanoparticle sample and 1-octene and summary of the emission spectral line widths of Ge nanoparticles reported elsewhere when compared to our data. 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
N.S. designed the experiment, carried out research, analyzed data, and wrote the article. D.H. carried out research. Y.M. helped to characterize the nanoparticles. Y.S. was involved in the experiment design. All authors discussed the results and commented on the article. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Nanotechnology Innovation Center for the use of its facilities. This work was financially supported by JSTPRESTO, JST A-step (no. AS221Z00791C), a grant-in-aid for challenging exploratory research (no. 23655138), MEXT, and H
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the Konica Minolta Imaging Science Award from the Konica Minolta Science and Technology Foundation, Japan.
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