Photoluminescence of Functionalized Germanium Nanocrystals

May 9, 2017 - Electrical and Computer Engineering, University of Delaware, Newark, Delaware 19716, United States. ‡ Department of Chemical and Biolo...
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Photoluminescence of Functionalized Germanium Nanocrystals Embedded in Arsenic Sulfide Glass Tingyi Gu,† Jia Gao,‡ Evgeny E. Ostroumov,§ Hyuncheol Jeong,‡ Fan Wu,∥ Romain Fardel,∥,# Nan Yao,∥ Rodney D. Priestley,‡,∥ Gregory D. Scholes,§,∥ Yueh-Lin Loo,‡,⊥ and Craig B. Arnold*,∥,# †

Electrical and Computer Engineering, University of Delaware, Newark, Delaware 19716, United States Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, United States § Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States ∥ Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, New Jersey 08544, United States ⊥ Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, United States # Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544, United States ‡

S Supporting Information *

ABSTRACT: Embedding metallic and semiconductor nanoparticles in a chalcogenide glass matrix effectively modifies the photonic properties. Such nanostructured materials could play an important role in optoelectronic devices, catalysis, and imaging applications. In this work, we fabricate and characterize germanium nanocrystals (Ge NCs) embedded in arsenic sulfide thin films by pulsed laser ablation in aliphatic amine solutions. Unstable surface termination of aliphatic groups and stable termination by amine on Ge NCs are indicated by Raman and Fourier-transform infrared spectroscopy measurements. A broad-band photoluminescence in the visible range is observed for the amine functionalized Ge NCs. A noticeable enhancement of fluorescence is observed for Ge NCs in arsenic sulfide, after annealing to remove the residual solvent of the glass matrix. KEYWORDS: laser ablation, solution process, nanocrystals, chalcogenide glass, photoluminescence

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INTRODUCTION Uniform embedding light-emitting materials into a passive media has a wide range of applications, such as active photonic devices, fluorescence ink for inkjet printing, label-free chemical detection, and fluorescent tagging in security devices to biomedical imaging.1−4 Semiconductor nanocrystals embedded in a matrix exhibit controllable light-emission properties by varying the size and distribution of nanoparticles, surface chemistry, and the matrix materials. Such features are essential for engineering active photonic components for integrated photonic devices.5−19 Earth abundance of germanium and tunable optoelectronics through interface engineering makes the current approach an attractive alternative to current luminescence ion doping for active devices. The optoelectronic properties of germanium nanocrystals (Ge NCs) are critically influenced by the fabrication technique, and extensive investigation shows that the electronic structure of Ge NCs is strongly modified by surface chemistry.20−26 Embedding Ge NCs into a matrix allows one to control that interface, and this can be accomplished by various techniques such as sputtering or ion implantation.27−30 Among different matrix materials, arsenic sulfide (As2S3) has a number of beneficial optical properties, such as high refractive © 2017 American Chemical Society

index and low phonon absorption, which make it useful for integrated photonic devices in the infrared and mid-infrared.31,32 Also, it has various properties suitable for matrix materials, such as low phonon density of states, high transparency in visible wavelength, and good chemical and thermal stabilities.33−35 Solution processing is a versatile method of making high refractive index As2S3 structures, which has shown great promise for integration and fabrication of photonic devices.36−41 In addition, it opens the possibility for introducing nanoparticles into the material through the solution phase. Among various techniques of producing Ge NCs, pulsed laser ablation in liquids (PLAL) allows precursor-free generation of high purity nanomaterials.42−44 In particular, previous studies showed that silver nanoparticles can be embedded into As2S3 using PLAL in aliphatic amine based chalcogenide solutions.45,46 In this paper, we report the formation and optoelectronic characterization of Ge NCs in As2S3 using PLAL in amine solutions of As2S3. The results of our study indicate that the Received: February 20, 2017 Accepted: May 9, 2017 Published: May 9, 2017 18911

DOI: 10.1021/acsami.7b02520 ACS Appl. Mater. Interfaces 2017, 9, 18911−18917

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ACS Applied Materials & Interfaces

Figure 1. Pulsed laser ablation in liquids (PLAL) of germanium nanocrystals (Ge NCs). (a) Optical setup for pulsed laser ablation for Ge target in As2S3 solution sealed in an oxygen free environment. (b−d) TEM images of Ge NCs in the glass, with increasing magnification as indicated by scale bar. (e) Characteristic particle size histogram obtained from (c) showing a mean particle diameter is 5.3 nm with a standard deviation of 3.4 nm (solid line), and a minor peak centered at 11 nm (dashed line). The particle density is approximately 4 × 103/μm2.

Figure 2. Optical spectroscopy of as-prepared Ge NCs. (a) Absorption spectra of solution processed As2S3 thin film (gray) and Ge NC embedded thin films (red). (b) PL spectra of films in (a). (c) EDX of As2S3 glass with embedded Ge NCs (red curve), compared to pure As2S3 sample (gray dashed curve). (d) Corresponding Raman spectra (633 nm laser excitation) of solution processed thin film of As2S3 (gray) and Ge nanoparticles doped samples (red).

particle diameter of 5.3 nm and a standard deviation of 3.4 nm (Figure 1 c). Figure 1d shows a typical crystalline structure of Ge embedded in the amorphous chalcogenide. The lattice fringes have a periodicity of 3.3 Å, consistent with the characteristic spacing in the [110] direction of Ge. Above 20 nm (Figure 1b), we begin to see signs of agglomeration as a small number of clusters composed of smaller particles can be found in the TEM image, but lower resolution SEM imaging (Supporting Information) reveals that such aggregates are welldispersed throughout the film. Figure 1e shows the particle size histogram of Ge NCs in the glass. A mean particle diameter of

amine solvent interacts with the Ge NCs surface, creating a surface termination of organic ligands from the amine solvent, that leads to broad-band photoluminescence (PL) in the visible spectral region.

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RESULTS AND DISCUSSION The scheme of PLAL of Ge NCs is shown in Figure 1a, with the complete details described in the Methods section. The Ge NCs produced by PLAL have arbitrary shapes and sizes, but remain crystalline. The density of Ge NCs (sized less than 20 nm as measured in TEM) is about 4 × 103/μm2, with a mean 18912

DOI: 10.1021/acsami.7b02520 ACS Appl. Mater. Interfaces 2017, 9, 18911−18917

Research Article

ACS Applied Materials & Interfaces

Figure 3. Evolution of PLAL Ge NCs. (a) FTIR absorption spectra of solution processed As2S3 thin film (blue), Ge NCs embedded As2S3 asprepared (red), aged in solution (gray), and annealed at 110 °C for 5 h (black). (b) PLE spectra of Ge NCs embedded in As2S3 at different detection wavelengths.

Figure 4. PL Ge NCs after annealing. (a) Ge NCs in PA and As2S3/PA, before and after baking at 110 °C. (b) The relative PL quantum yield in Ge NCs in propylamine and As2S3, before and after baking to remove excess solvent. Inset: Energy level diagram of As2S3 matrix and Ge. (c) PL dynamics of the excitation laser (gray), Ge NCs/PA (blue), and Ge NCs/As2S3/PA (red). Solid lines represent biexponential fit.

5.3 nm is obtained by curve fitting of the measured histogram to a Gaussian distribution. The second peak near 11 nm mean size represents the larger germanium flakes peaked off during the laser machining processing. The absorption and fluorescence properties of Ge NCs embedded As2S3 films are characterized in Figure 2. Figure 2a compares the UV−vis spectrum of Ge NCs embedded As2S3 film (red curve) and matrix As2S3 material (gray curve). The addition of Ge NCs at the density given in Figure 1 extends the absorption edge of the doped sample to the near-infrared, which is typical for materials with lower band gap than As2S3. We compare the fluorescence for the pristine and Ge NCs embedded As2S3 thin films at the same laser excitation wavelength (532 nm, CW) and power levels. The excitation laser is adjusted to a low enough intensity to avoid any nonlinear response of the material. Pristine samples show no fluorescence (gray line in Figure 2b), whereas broad-band emission is observed in the Ge NCs embedded samples (red curve in Figure 2b). As a control sample, laser ablation of Ge in acetone and aliphatic amine solvent was performed under the same PLAL conditions (Figure S3). In acetone, particle aggregation is observed, and consequently, the deposited samples exhibit low light emission which is not easily detected by the spectrometer. In the amine control sample, we find a PL spectrum that is like the PL from the Ge NCs embedded As2S3 without the additional As2S3 peaks. Therefore, we conclude that the observed PL spectrum in Figure 2b is due to the interaction of the Ge with the amine and not due to the specific matrix material.

The different elemental components of the doped sample compared to the pristine As2S3 film are identified by the EDX spectra (Figure 2c). Both As and Ge have electron energies near 1.29 eV, and thus, it is difficult to differentiate these two. However, the normalized spectrum (to the intensity of sulfide peak) shows that Ge NCs increase the intensity of the As/Ge peak by 53%, representing effective incorporation of Ge NCs into the As2S3 matrix, which implies a total Ge concentration of ∼34% in the sample under inspection. Differences in the Raman spectra correspond to the extra Ge species in the As2S3 glass structure (Figure 2d), where a new peak associated with the Ge−Ge bond near 300 cm−1 is observed for the embedded samples. Fourier-transform infrared spectroscopy (FTIR) is further used to identify the chemical composition of the doped chalcogenide. As a control sample, we measured the FTIR spectrum of solution processed As2S3 without Ge NCs (blue curve in Figure 3a). The PLAL prepared Ge NCs embedded As2S3 films (red curve in Figure 3a) show a new peak around 670 cm−1, representing the stretching mode of a Ge−N bond in the HGeNH2 molecule.48 The signals from the Ge−N bond and bending mode of NH3 in GeNH3 (near 1120 cm−1) become stronger as the PLAL solution is aged for a week in an oxygen free environment before drop-casting the film (the gray curve in Figure 3a) and remain even after baking the films at 110 °C for 5 h (black curve in Figure 3a) and remains even after baking the films at 110 °C for 5 h (black curve in Figure 3a). It has been previously shown that PLAL in n-propylamine (PA) creates ammonium ions (−NH3+) and amine (−NH2). 18913

DOI: 10.1021/acsami.7b02520 ACS Appl. Mater. Interfaces 2017, 9, 18911−18917

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Figure 5. Time-resolved PL decay of functionalized Ge NCs. (a) PL decay dynamics for the Ge NCs in PA at emission wavelengths of 550, 575, 600, and 700 nm. The excitation wavelength is set at 507 nm. (b) The dominant PL decay constant.

ing Information). The ionization energy is measured to be −5.4 eV, and the optical band gap of 2.68 eV is estimated from absorption spectroscopy measurements. We then derive the electron affinity of As2S3 to be −3.9 eV. A schematic of the energy level diagram of As2S3 and Ge is plotted as an inset in Figure 4b. The PL QY of the Ge NCs is obtained by normalizing the PL intensity by its absorbance, and we compare the QY of the four Ge NCs samples (Figure 4b). Annealing at 110 °C in vacuum for 5 h reduces the residual solvent component in the As2S3 matrix, and thus the absorption from the molecular vibrational modes of organic ligands. We observed that, without As2S3, the QY increased slightly after annealing, whereas annealing in the presence of the As2S3 leads to an increase in the QY by an order of magnitude. The improvement of QY might also be attributed to the reduction of the solvent components in the As2S3 matrix, as verified by the FTIR absorption peak intensity of aliphatic groups (Figure 3a). The enhancement of PL intensity is examined by timeresolved measurements (Figure 4c). The excitation laser was set at 507 nm with pulses of less than 1 ns duration, and the PL decay was measured at 540 nm. We apply a biexponential function to fit the PL decay curves. A dominant component is found for the longer time constant. We believe the shorter time constant component might be related to the optical activation of photocarriers in the germanium crystal, followed by rapid carrier trapping on the surface of Ge NPs, and the longer components are more likely related to the interband relaxation of the photocarriers from ground states on the surface of Ge NPs. The PL decay constant of Ge NCs prepared in PA is found to be 17 ns (solid blue line in Figure 4c) and increases to 160 ns for the annealed samples in the As2S3 matrix (red solid line in Figure 4c). The emission wavelength dependent carrier dynamics has been reported,12,54−56 and here, we present a systematic study and found its lifetime spectrum of the slower component is similar to the photoluminescence spectrum measured in a steady state. We measured the time-resolved PL at different emission wavelengths (Figure 5a). Theoretically, the PL lifetime is determined by both radiative and nonradiative processes: τ = 1/(kr + knr). Free excitonic PL intensity increases with the increase in the nonradiative PL lifetime (1/knr).54 The longest decay constant was measured to be 23 ns at 570 nm, with 507 nm laser excitation. The average lifetimes originated from different defects states at different emission wavelengths, and decreased to less than 5 ns at longer wavelengths near 700 nm (Figure 5b). The wavelength dependent temporal decay

The ion appears in as-prepared samples but decays with time, whereas the amine remains in the sample.47 The chemical reaction leading to the observed signal in our sample is associated with the interaction of the solvent and surface of Ge NCs; e.g., the amine nitrogen reacts with the electropositive Ge ions as a Lewis base.48 The tetrathiogermanate ions in solution are compensated by C3H7NH3+ ions.49 Similar broad-band PL can be repeated for Ge NCs in ethylamine, propylamine, and amylamine but absent in the series of solutions without Ge NCs (Figure S3). The solvent peaks between 2800 and 3000 cm−1, which are assigned to N-H stretch and aliphatic C-H stretches, significantly decreased after annealing. The As2S3+x(C3H7NH3+)2x are decomposed to As2S3 after baking.50−52 The amorphous states on the surface of Ge NCs turn out to be the major contributor the PL signal, as verified by photoluminescence excitation (PLE) measurements in a Ge NCs sample exhibiting good PL quantum yield (QY) (Figure 3b). At fixed emission wavelengths of 680, 700, 720, and 740 nm, PLE spectra exhibit a trend of red shifting to longer wavelength. The energy levels of peaks on PLE spectra are below the absorption edge of the As2S3 matrix, and within the band tail states of the Ge NCs (measured in Figure 1a). At a fixed luminescent wavelength, the PLE spectra show the related excited states directly donating excitons/carriers into the probed luminescent state. As PLE for ordered crystalline germanium is usually independent of its PL wavelength, the PL signal mostly comes from surface states of Ge NCs. The PL spectra of Ge NCs in and out of an As2S3 matrix are shown in Figure 4a. The blue arrows denote the peak position for the four samples, showing a red shift for Ge NCs created in PA solvent compared to the one in the As2S3 matrix. The line shape of PL remains unchanged after heating, while the intensity of baked samples in the As2S3 matrix is dramatically increased after annealing, given the removal of residual solvent from the matrix (Figure 4b). The solvent residue could lead to increased nonradiative recombination in the matrix, and thus significantly reduce the emission quantum yield Φ = kr/(knr + kr(λ)), where kr indicates the radiative recombination rate and knr is the nonradiative rate. As photoluminescence from solution processed As2S3 thin film is negligibly weak53 and band-to-band transitions of Ge NCs are at much longer wavelengths, the radiative recombination most likely originates from trap/defects states on the surface of Ge NCs, and thus exhibits a broad distribution of photon energy. The ionization energy and optical band gap of the As2S3 matrix have been measured using an ambient photoelectron spectrometer and UV−vis absorption spectroscopy (Support18914

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constant is proportional to its PL intensity measured at steady state (Ge NCs in PA PL spectrum in Figure 4a).

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Tingyi Gu: 0000-0003-0414-6333 Rodney D. Priestley: 0000-0001-6765-2933 Gregory D. Scholes: 0000-0003-3336-7960 Yueh-Lin Loo: 0000-0002-4284-0847

CONCLUSIONS We report the doping of Ge NCs in an amorphous As2S3 glass matrix, by PLAL in aliphatic amine solvents. The PLAL process creates aliphatic and amine groups from the solvent, which terminate the surface of Ge NCs synthesized in this manner. The amine terminated surface minimizes Ge NCs’ agglomeration in glass, and improves the quantum yield of PL. We observe broad-band PL in the visible range with lifetimes on the scale of several nanoseconds. PLAL offers a straightforward and practical way of producing Ge NCs doped chalcogenide for active components for a variety of applications.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for experimental assistance from REU students, B. Abdi and T. Keute, and discussions with B. P. Rand of Princeton University, and N. Ge and H. Holder of HPI laboratories. Funding from the National Science Foundation (NSF) is gratefully acknowledged (Grant EEC-0540832 and Grant DMR-1420541). J.G. acknowledge funding from the NSF through Grant No. CHE-1124754, as well as from NRI (Gift No. 2011-NE-2205GB) under its joint initiative “Nanoelectronics Beyond 2020” with the NSF.

METHODS

Preparation of Ge-NCs. Arsenic sulfide (As2S3) solution was prepared by dissolving As2S3 powder into n-propylamine at a concentration of 0.8 mol/L. A Teflon cell was filled with the As2S3 solution, and transferred into an airtight chamber in a nitrogen-filled glovebox where a germanium (Ge) target was placed on the bottom of the cell. For the control studies, a Ge target was placed in a cuvette with acetone. The chamber/cuvette was positioned close to the focus of a neodymium-doped yttrium aluminum garnet (Nd:YAG) pulsed laser beam (1064 nm, 30 ps decay length, repetition rate of 30 Hz, 1 mJ/pulse). At the end of the experiment, the chamber/cuvette was disassembled in the glovebox and the nanoparticle containing liquid was collected from the cell with a pipet. Large particles in the solution were removed by filtering the solution through a 200 nm pore size syringe filter. Characterization. Samples for transmission electron microscopy (TEM) were prepared in a glovebox by drop-casting the solution onto carbon-coated copper grids. Images were collected on a CM200 electron microscope operating at 200 kV. Average particles diameters and standard deviations were determined by counting 240 particles. The solution was drop-casted onto a silicon wafer for scanning electron microscopy (SEM) measurements and energy-dispersive Xray spectroscopy (EDX) inspection, and spin-coated onto a glass substrate for optical absorption measurements. The absorption spectra of the chalcogenide glass films on silica substrates are obtained on an Ocean Optics HR4000 high-resolution spectrometer. Raman and photoluminescence spectra were obtained on a Horiba micro Raman Spectrometer with a 100× objective. The ionization potentials of the intrinsic and the doped As2S3 were measured using a photoelectron spectrometer (Riken Keiki Model AC-2). This instrument works in a nitrogen atmosphere at atmospheric pressure. The electron detector of this tool is an open counter, which sits above the sample, and is capable of detecting a broad energy range (3.4−6.3 eV). Fourier transform infrared spectroscopy (FTIR) measurements were performed by a N2-purged Nicolet iS50 FTIR spectrometer. Water and CO2 traces were removed from the raw data, and a baseline correction was performed.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02520. Experimental methods, including the preparation of GeNCs and characterizations; formation of germanium sulfide bond confirmed by PL, Raman, and absorption spectra (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 18915

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DOI: 10.1021/acsami.7b02520 ACS Appl. Mater. Interfaces 2017, 9, 18911−18917

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DOI: 10.1021/acsami.7b02520 ACS Appl. Mater. Interfaces 2017, 9, 18911−18917