Photoluminescence of Functionalized Germanium NanocrystalsEmbedded in Arsenic Sulfide GlassTingyi 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 nano-particles in a chalcogenide glass matrix effectively modifies thephotonic properties. Such nanostructured materials could playan important role in optoelectronic devices, catalysis, andimaging applications. In this work, we fabricate and character-ize germanium nanocrystals (Ge NCs) embedded in arsenicsulfide thin films by pulsed laser ablation in aliphatic aminesolutions. Unstable surface termination of aliphatic groups andstable termination by amine on Ge NCs are indicated byRaman and Fourier-transform infrared spectroscopy measure-ments. A broad-band photoluminescence in the visible range isobserved for the amine functionalized Ge NCs. A noticeable enhancement of fluorescence is observed for Ge NCs in arsenicsulfide, after annealing to remove the residual solvent of the glass matrix.

KEYWORDS: laser ablation, solution process, nanocrystals, chalcogenide glass, photoluminescence

■ INTRODUCTION

Uniform embedding light-emitting materials into a passivemedia has a wide range of applications, such as active photonicdevices, fluorescence ink for inkjet printing, label-free chemicaldetection, and fluorescent tagging in security devices tobiomedical imaging.1−4 Semiconductor nanocrystals embeddedin a matrix exhibit controllable light-emission properties byvarying the size and distribution of nanoparticles, surfacechemistry, and the matrix materials. Such features are essentialfor engineering active photonic components for integratedphotonic devices.5−19 Earth abundance of germanium andtunable optoelectronics through interface engineering makesthe current approach an attractive alternative to currentluminescence ion doping for active devices. The optoelectronicproperties of germanium nanocrystals (Ge NCs) are criticallyinfluenced by the fabrication technique, and extensiveinvestigation shows that the electronic structure of Ge NCs isstrongly modified by surface chemistry.20−26 Embedding GeNCs into a matrix allows one to control that interface, and thiscan be accomplished by various techniques such as sputteringor ion implantation.27−30

Among different matrix materials, arsenic sulfide (As2S3) hasa number of beneficial optical properties, such as high refractive

index and low phonon absorption, which make it useful forintegrated photonic devices in the infrared and mid-infra-red.31,32 Also, it has various properties suitable for matrixmaterials, such as low phonon density of states, hightransparency in visible wavelength, and good chemical andthermal stabilities.33−35 Solution processing is a versatilemethod of making high refractive index As2S3 structures,which has shown great promise for integration and fabricationof photonic devices.36−41 In addition, it opens the possibility forintroducing nanoparticles into the material through the solutionphase. Among various techniques of producing Ge NCs, pulsedlaser ablation in liquids (PLAL) allows precursor-freegeneration of high purity nanomaterials.42−44 In particular,previous studies showed that silver nanoparticles can beembedded into As2S3 using PLAL in aliphatic amine basedchalcogenide solutions.45,46

In this paper, we report the formation and optoelectroniccharacterization of Ge NCs in As2S3 using PLAL in aminesolutions of As2S3. The results of our study indicate that the

Received: February 20, 2017Accepted: May 9, 2017Published: May 9, 2017

Research Article

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© 2017 American Chemical Society 18911 DOI: 10.1021/acsami.7b02520ACS Appl. Mater. Interfaces 2017, 9, 18911−18917

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

■ RESULTS AND DISCUSSION

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

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

Figure 1. Pulsed laser ablation in liquids (PLAL) of germanium nanocrystals (Ge NCs). (a) Optical setup for pulsed laser ablation for Ge target inAs2S3 solution sealed in an oxygen free environment. (b−d) TEM images of Ge NCs in the glass, with increasing magnification as indicated by scalebar. (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 embeddedthin 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 (graydashed curve). (d) Corresponding Raman spectra (633 nm laser excitation) of solution processed thin film of As2S3 (gray) and Ge nanoparticlesdoped samples (red).

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5.3 nm is obtained by curve fitting of the measured histogramto a Gaussian distribution. The second peak near 11 nm meansize represents the larger germanium flakes peaked off duringthe laser machining processing.The absorption and fluorescence properties of Ge NCs

embedded As2S3 films are characterized in Figure 2. Figure 2acompares the UV−vis spectrum of Ge NCs embedded As2S3film (red curve) and matrix As2S3 material (gray curve). Theaddition of Ge NCs at the density given in Figure 1 extends theabsorption 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 NCsembedded As2S3 thin films at the same laser excitationwavelength (532 nm, CW) and power levels. The excitationlaser is adjusted to a low enough intensity to avoid anynonlinear response of the material. Pristine samples show nofluorescence (gray line in Figure 2b), whereas broad-bandemission is observed in the Ge NCs embedded samples (redcurve in Figure 2b). As a control sample, laser ablation of Ge inacetone and aliphatic amine solvent was performed under thesame PLAL conditions (Figure S3). In acetone, particleaggregation is observed, and consequently, the depositedsamples exhibit low light emission which is not easily detectedby the spectrometer. In the amine control sample, we find a PLspectrum that is like the PL from the Ge NCs embedded As2S3without the additional As2S3 peaks. Therefore, we conclude thatthe observed PL spectrum in Figure 2b is due to the interactionof the Ge with the amine and not due to the specific matrixmaterial.

The different elemental components of the doped samplecompared to the pristine As2S3 film are identified by the EDXspectra (Figure 2c). Both As and Ge have electron energiesnear 1.29 eV, and thus, it is difficult to differentiate these two.However, the normalized spectrum (to the intensity of sulfidepeak) shows that Ge NCs increase the intensity of the As/Gepeak by 53%, representing effective incorporation of Ge NCsinto the As2S3 matrix, which implies a total Ge concentration of∼34% in the sample under inspection. Differences in theRaman spectra correspond to the extra Ge species in the As2S3glass structure (Figure 2d), where a new peak associated withthe Ge−Ge bond near 300 cm−1 is observed for the embeddedsamples.Fourier-transform infrared spectroscopy (FTIR) is further

used to identify the chemical composition of the dopedchalcogenide. As a control sample, we measured the FTIRspectrum of solution processed As2S3 without Ge NCs (bluecurve in Figure 3a). The PLAL prepared Ge NCs embeddedAs2S3 films (red curve in Figure 3a) show a new peak around670 cm−1, representing the stretching mode of a Ge−N bond inthe HGeNH2 molecule.48 The signals from the Ge−N bondand bending mode of NH3 in GeNH3 (near 1120 cm−1)become stronger as the PLAL solution is aged for a week in anoxygen free environment before drop-casting the film (the graycurve in Figure 3a) and remain even after baking the films at110 °C for 5 h (black curve in Figure 3a) and remains evenafter baking the films at 110 °C for 5 h (black curve in Figure3a).It has been previously shown that PLAL in n-propylamine

(PA) creates ammonium ions (−NH3+) and amine (−NH2).

Figure 3. Evolution of PLAL Ge NCs. (a) FTIR absorption spectra of solution processed As2S3 thin film (blue), Ge NCs embedded As2S3 as-prepared (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 detectionwavelengths.

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 GeNCs in propylamine and As2S3, before and after baking to remove excess solvent. Inset: Energy level diagram of As2S3 matrix and Ge. (c) PLdynamics of the excitation laser (gray), Ge NCs/PA (blue), and Ge NCs/As2S3/PA (red). Solid lines represent biexponential fit.

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The ion appears in as-prepared samples but decays with time,whereas the amine remains in the sample.47 The chemicalreaction leading to the observed signal in our sample isassociated with the interaction of the solvent and surface of GeNCs; e.g., the amine nitrogen reacts with the electropositive Geions as a Lewis base.48 The tetrathiogermanate ions in solutionare compensated by C3H7NH3

+ ions.49 Similar broad-band PLcan be repeated for Ge NCs in ethylamine, propylamine, andamylamine but absent in the series of solutions without GeNCs (Figure S3). The solvent peaks between 2800 and 3000cm−1, which are assigned to N-H stretch and aliphatic C-Hstretches, 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 themajor contributor the PL signal, as verified by photo-luminescence excitation (PLE) measurements in a Ge NCssample exhibiting good PL quantum yield (QY) (Figure 3b). Atfixed emission wavelengths of 680, 700, 720, and 740 nm, PLEspectra exhibit a trend of red shifting to longer wavelength. Theenergy levels of peaks on PLE spectra are below the absorptionedge of the As2S3 matrix, and within the band tail states of theGe NCs (measured in Figure 1a). At a fixed luminescentwavelength, the PLE spectra show the related excited statesdirectly donating excitons/carriers into the probed luminescentstate. As PLE for ordered crystalline germanium is usuallyindependent of its PL wavelength, the PL signal mostly comesfrom 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 positionfor the four samples, showing a red shift for Ge NCs created inPA solvent compared to the one in the As2S3 matrix. The lineshape of PL remains unchanged after heating, while theintensity of baked samples in the As2S3 matrix is dramaticallyincreased after annealing, given the removal of residual solventfrom the matrix (Figure 4b). The solvent residue could lead toincreased nonradiative recombination in the matrix, and thussignificantly reduce the emission quantum yield Φ = kr/(knr +kr(λ)), where kr indicates the radiative recombination rate andknr is the nonradiative rate. As photoluminescence fromsolution processed As2S3 thin film is negligibly weak53 andband-to-band transitions of Ge NCs are at much longerwavelengths, the radiative recombination most likely originatesfrom trap/defects states on the surface of Ge NCs, and thusexhibits a broad distribution of photon energy.The ionization energy and optical band gap of the As2S3

matrix have been measured using an ambient photoelectronspectrometer and UV−vis absorption spectroscopy (Support-

ing Information). The ionization energy is measured to be −5.4eV, and the optical band gap of 2.68 eV is estimated fromabsorption spectroscopy measurements. We then derive theelectron affinity of As2S3 to be −3.9 eV. A schematic of theenergy level diagram of As2S3 and Ge is plotted as an inset inFigure 4b. The PL QY of the Ge NCs is obtained bynormalizing the PL intensity by its absorbance, and we comparethe QY of the four Ge NCs samples (Figure 4b). Annealing at110 °C in vacuum for 5 h reduces the residual solventcomponent in the As2S3 matrix, and thus the absorption fromthe molecular vibrational modes of organic ligands. Weobserved that, without As2S3, the QY increased slightly afterannealing, whereas annealing in the presence of the As2S3 leadsto an increase in the QY by an order of magnitude. Theimprovement of QY might also be attributed to the reductionof the solvent components in the As2S3 matrix, as verified bythe FTIR absorption peak intensity of aliphatic groups (Figure3a).The enhancement of PL intensity is examined by time-

resolved measurements (Figure 4c). The excitation laser wasset at 507 nm with pulses of less than 1 ns duration, and the PLdecay was measured at 540 nm. We apply a biexponentialfunction to fit the PL decay curves. A dominant component isfound for the longer time constant. We believe the shorter timeconstant component might be related to the optical activationof photocarriers in the germanium crystal, followed by rapidcarrier trapping on the surface of Ge NPs, and the longercomponents are more likely related to the interband relaxationof the photocarriers from ground states on the surface of GeNPs. The PL decay constant of Ge NCs prepared in PA isfound to be 17 ns (solid blue line in Figure 4c) and increases to160 ns for the annealed samples in the As2S3 matrix (red solidline in Figure 4c). The emission wavelength dependent carrierdynamics has been reported,12,54−56 and here, we present asystematic study and found its lifetime spectrum of the slowercomponent is similar to the photoluminescence spectrummeasured in a steady state.We measured the time-resolved PL at different emission

wavelengths (Figure 5a). Theoretically, the PL lifetime isdetermined by both radiative and nonradiative processes: τ =1/(kr + knr). Free excitonic PL intensity increases with theincrease in the nonradiative PL lifetime (1/knr).

54 The longestdecay constant was measured to be 23 ns at 570 nm, with 507nm laser excitation. The average lifetimes originated fromdifferent defects states at different emission wavelengths, anddecreased to less than 5 ns at longer wavelengths near 700 nm(Figure 5b). The wavelength dependent temporal decay

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.

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

■ CONCLUSIONSWe report the doping of Ge NCs in an amorphous As2S3 glassmatrix, by PLAL in aliphatic amine solvents. The PLAL processcreates aliphatic and amine groups from the solvent, whichterminate the surface of Ge NCs synthesized in this manner.The amine terminated surface minimizes Ge NCs’ agglomer-ation in glass, and improves the quantum yield of PL. Weobserve broad-band PL in the visible range with lifetimes on thescale of several nanoseconds. PLAL offers a straightforward andpractical way of producing Ge NCs doped chalcogenide foractive components for a variety of applications.

■ METHODSPreparation of Ge-NCs. Arsenic sulfide (As2S3) solution was

prepared by dissolving As2S3 powder into n-propylamine at aconcentration of 0.8 mol/L. A Teflon cell was filled with the As2S3solution, and transferred into an airtight chamber in a nitrogen-filledglovebox where a germanium (Ge) target was placed on the bottom ofthe cell. For the control studies, a Ge target was placed in a cuvettewith acetone. The chamber/cuvette was positioned close to the focusof a neodymium-doped yttrium aluminum garnet (Nd:YAG) pulsedlaser beam (1064 nm, 30 ps decay length, repetition rate of 30 Hz, 1mJ/pulse). At the end of the experiment, the chamber/cuvette wasdisassembled in the glovebox and the nanoparticle containing liquidwas collected from the cell with a pipet. Large particles in the solutionwere removed by filtering the solution through a 200 nm pore sizesyringe filter.Characterization. Samples for transmission electron microscopy

(TEM) were prepared in a glovebox by drop-casting the solution ontocarbon-coated copper grids. Images were collected on a CM200electron microscope operating at 200 kV. Average particles diametersand standard deviations were determined by counting 240 particles.The solution was drop-casted onto a silicon wafer for scanningelectron microscopy (SEM) measurements and energy-dispersive X-ray spectroscopy (EDX) inspection, and spin-coated onto a glasssubstrate for optical absorption measurements. The absorption spectraof the chalcogenide glass films on silica substrates are obtained on anOcean Optics HR4000 high-resolution spectrometer.Raman and photoluminescence spectra were obtained on a Horiba

micro Raman Spectrometer with a 100× objective. The ionizationpotentials of the intrinsic and the doped As2S3 were measured using aphotoelectron spectrometer (Riken Keiki Model AC-2). Thisinstrument works in a nitrogen atmosphere at atmospheric pressure.The electron detector of this tool is an open counter, which sits abovethe sample, and is capable of detecting a broad energy range (3.4−6.3eV).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 baselinecorrection was performed.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.7b02520.

Experimental methods, including the preparation of Ge-NCs and characterizations; formation of germaniumsulfide bond confirmed by PL, Raman, and absorptionspectra (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: cbarnold@princeton.edu.

ORCID

Tingyi Gu: 0000-0003-0414-6333Rodney D. Priestley: 0000-0001-6765-2933Gregory D. Scholes: 0000-0003-3336-7960Yueh-Lin Loo: 0000-0002-4284-0847NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

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

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