Fabrication of uniformly dispersed nanoparticle-doped chalcogenide glassChao Lu, Juliana M. P. Almeida, Nan Yao, and Craig Arnold Citation: Applied Physics Letters 105, 261906 (2014); doi: 10.1063/1.4905283 View online: http://dx.doi.org/10.1063/1.4905283 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/26?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Mineralization and optical characterization of copper oxide nanoparticles using a high aspect ratio bio-template J. Appl. Phys. 116, 154308 (2014); 10.1063/1.4898809 Effects of surface functional groups on the formation of nanoparticle-protein corona Appl. Phys. Lett. 101, 263701 (2012); 10.1063/1.4772509 Photoexpansion and nano-lenslet formation in amorphous As2S3 thin films by 800 nm femtosecond laserirradiation J. Appl. Phys. 112, 033105 (2012); 10.1063/1.4745021 Effect of cluster size of chalcogenide glass nanocolloidal solutions on the surface morphology of spin-coatedamorphous films J. Appl. Phys. 103, 063511 (2008); 10.1063/1.2895005 Structure and grain growth of Ti O 2 nanoparticles investigated by electron and x-ray diffractions and Ta 181perturbed angular correlations J. Appl. Phys. 100, 024305 (2006); 10.1063/1.2214182

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Fabrication of uniformly dispersed nanoparticle-doped chalcogenide glass

Chao Lu,1,2 Juliana M. P. Almeida,1,3 Nan Yao,2 and Craig Arnold1,2,a)

1Department of Mechanical and Aerospace Engineering, Princeton, New Jersey 08544, USA2Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton,New Jersey 08544, USA3S~ao Carlos Institute of Physics, University of S~ao Paulo, 13560 S~ao Carlos, SP, Brazil

(Received 17 October 2014; accepted 17 December 2014; published online 30 December 2014)

The dispersion of metallic nanoparticles within a chalcogenide glass matrix has the potential for

many important applications in active and passive optical materials. However, the challenge of

particle agglomeration, which can occur during traditional thin film processing, leads to materials

with poor performance. Here, we report on the preparation of a uniformly dispersed

Ag-nanoparticle (Ag NP)/chalcogenide glass heterogeneous material prepared through a combined

laser- and solution-based process. Laser ablation of bulk silver is performed directly within an

arsenic sulfide/propylamine solution resulting in the formation of Ag NPs in solution with an

average particle size of less than 15 nm as determined by dynamic light scattering. The prepared

solutions are fabricated into thin films using standard coating processes and are then analyzed using

energy-dispersive X-ray spectroscopy and transmission electron microscopy to investigate the

particle shape and size distribution. By calculating the nearest neighbor index and standard normal

deviate of the nanoparticle locations inside the films, we verify that a uniformly dispersed

distribution is achieved through this process. VC 2014 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4905283]

Photodarkening, photobleaching, and photodoping

are well-known light-induced phenomena in chalcogenide

glasses.1–3 The photodoping effect has drawn much attention

recently due to its potential application in various areas, such

as mid-infrared communications, holographic data storage,

diffractive elements, and sensing devices.4–6 However, in

conventional photodoping procedures, which rely on thermal

evaporation and sputtering to create a thin layer of metal on

top of the chalcogenide film, the thickness of the doped layer

is limited by the diffusion depth of the silver.7 Also the

uniformity of the doped layer is difficult to control, thereby

setting limitations in fabricating application-favorable bulk

structures.

One possible method to resolve these difficulties in fab-

ricating chalcogenide films is to employ a solution-based

process.8 In particular, by incorporating nanoparticles into

the deposited films, we can hope to achieve a uniform distri-

bution of silver throughout the entire depth, rather than

having silver concentrated only at the surface. Previous

researchers have shown the ability to distribution quantum

dots into chalcogenide systems using solution-based meth-

ods.9,10 In addition, these approaches can readily realize

large area or large thickness films, and the same solutions

can also be adopted for other precision dispensing techniques

such as mold casting, ink jet or laser direct write, allowing

spatial control over the added material. Nevertheless, uni-

form doping of silver nanoparticles (Ag NPs) into a chalco-

genide glass matrix without agglomeration remains

challenging, due to the tendency of Ag NPs to aggregate.

In this paper, we present experimental results of fabri-

cating uniformly dispersed nanoparticle-doped chalcogenide

glass using laser ablation11,12 and solution processing

methods.13,14 In the fabrication process, we first focus a

pulsed laser beam onto the surface of a bulk metallic sample

within an arsenic sulfide/propylamine solution and ablate the

material. The ejecta expandby carrying out the nanoparticle

generation steps directly in the solution of interest instead of

ideal solvents such as water or ethanol, the laser ablation

methods into the liquid solution, and condenses into in a sus-

pension of silver nanoparticles. In contrast to prior studies

which use ideal and/or pure solvents such as water or etha-

nol, or other organics,15–17 we perform this process directly

in the glass solution of interest. Ag-doped chalcogenide films

are then fabricated by spin-coating the resulting solution.

The prepared solution and films are analyzed using UV-Vis

spectroscopy, dynamic light scattering (DLS), energy-

dispersive X-ray spectroscopy (EDX), and transmission

electron microscopy (TEM) to investigate the particle struc-

ture and distribution. The results demonstrate that this pro-

cess can avoid the aggregation and additional processing

steps associated with other nanoparticles generation techni-

ques, such as wet chemical approaches,18 and a uniformly

dispersed nanoparticle-doped chalcogenide glass has been

fabricated.

Arsenic sulfide (As2S3) solution was prepared by grind-

ing bulk As2S3 pieces into a fine powder and dissolved into

n-propylamine solvent at a concentration of 0.8 mol/l.

Dissolution was carried out inside a sealed glass chamber to

prevent solvent evaporation. The dissolving process usually

took more than three days and a magnetic stirrer was used to

expedite this process. Exposure of solution to atmospheric

moisture was kept to a minimum throughout preparation pro-

cedure since water can lead to precipitate formation.19

To obtain the nanoparticles in As2S3 solution, we used

the experimental setup shown in Fig. 1. A silver target

(>99.99% purity, thickness 5.0 mm) was placed at thea)cbarnold@princeton.edu

0003-6951/2014/105(26)/261906/4/$30.00 VC 2014 AIP Publishing LLC105, 261906-1

APPLIED PHYSICS LETTERS 105, 261906 (2014)

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bottom of a Teflon cell, which was filled with As2S3 solution

afterwards. The silver target was covered by a layer of

solution 4 mm deep. The cell was then transferred into an

air-tight vacuum chamber. All these operations were carried

out inside a nitrogen-filled glovebox, to avoid the influence

of oxygen.

The chamber was then placed under the focus of a 30 ps,

1000 Hz, 1064 nm Q-Switched Nd:YAG laser beam, with

maximum energy of 1 mJ/pulse. The Gaussian laser beam

was focused, through the liquid layer, onto the target surface

using a lens with a focal length of 50 mm. Since absorption

by the arsenic sulfide solution was very small at the operat-

ing wavelength, the ablation beam passed through without

significant loss. The position of the target relative to the laser

beam was controlled by an X-Y-Z stage to maintain proper

focus and to avoid deep hole drilling in the target. Ablation

was performed for 10, 25, and 40 min, and we denote the

obtained solutions as 10-, 25-, and 40-min solutions in this

paper, respectively. Ablation was accompanied by the emis-

sion of plasma light from the surface, and bubbling was

observed immediately upon irradiation. The solution gradu-

ally turned brownish-yellow as the concentration of Ag NPs

increased. Finally, the chamber was moved back to the

glovebox, where the solution was collected.

Immediately after synthesis of the solutions, UV-Vis

absorption measurements of silver were conducted using an

Ocean Optics HR4000 high-resolution spectrometer. In order

to characterize morphology in thin films, TEM experiments

were carried out on a Philips CM200 microscope operating

at 200 kV. High resolution images were acquired using a

Gatan Orius 200 camera. Samples for TEM characterization

were prepared by pouring a droplet of the colloidal solution

onto 400-mesh copper grids covered with a holey carbon

film (from Ted Pella). The excess solvent was then allowed

to evaporate.

These suspensions were then deposited onto silicon

wafers for SEM and EDX analysis. Specifically, the solution

was first pipetted onto a silicon substrate, and the substrate

was immediately spun at 1000 rpm for 30 s. Resulting films

were soft-baked under vacuum at 60 �C for 1 h to remove

most of the solvent and then hard-baked at 180 �C to remove

excess solvent and to further densify the glass.20 Films pre-

pared under these conditions typically were approximately

1 lm thick and ready for subsequent analysis.

As photos of the generated Ag NPs solution in Fig. 2

(inset) show the color of these solutions gets noticeably

darker due to the increasing Ag NPs concentration resulting

from higher number of laser shots. Absorption of these solu-

tions is measured by UV-Vis spectroscopy using arsenic

sulfide solution as a control. In the results shown in Fig. 2,

different curves denote spectra from solutions experiencing

different ablation duration. The existence of Ag NPs introdu-

ces surface plasmon resonance bands, but due to the absorp-

tion of the As2S3 solution, only tails of these resonance

bands are observable. Moreover, these measurements dis-

close a redshift behavior of the absorption edge associated

with the increasing concentration of Ag NPs, which agrees

well with the literature.21–23

UV-Vis measurements indicate the existence of Ag NPs

without revealing information about size of those Ag NPs. In

order to determine the size distribution of the Ag NPs, DLS

measurements are performed on the 40-min solution. The

signal from pure As2S3 solution is also acquired for compari-

son. As shown in Fig. 3, background from As2S3 peaks

around 8 nm, which originate from the dissolved As2S3

units.20 The peak from Ag NPs solutions is centered around

14 nm, which reveals the average size of the majority of

nanoparticles. The DLS measurements are also performed on

10- and 25-min solutions, and the same distribution of Ag

NPs are obtained, revealing the size is independent with

ablation time. These values are in good agreement with the

literature,24,25 where nanoparticles average size of 5–25 nm

are generated in ideal solutions like water.

Measurements on the solution phase demonstrate that

the size of the nanoparticles is independent of ablation time,

while the concentration of particles is clearly rising with

increasing ablation time. However, the ultimate test of the

particle distribution is the fabrication of a film with a uni-

form dispersion of nanoparticles. Fig. 4 shows EDX results

FIG. 1. Schematic diagram of the experimental setup.

FIG. 2. Photos (inset) and UV-Vis absorbance spectrum of solution gener-

ated with different ablation duration. As the ablation time increases, the

color of the solutions changes from yellow to brown, indicating the absorp-

tion edge is red-shifted.

261906-2 Lu et al. Appl. Phys. Lett. 105, 261906 (2014)

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128.112.142.131 On: Wed, 25 Feb 2015 21:24:55

from spin-coated films using the 40-min solution. The rela-

tive weight of silver is measured to be 1.3%. Armed with

this value, the concentration of As2S3 solution (2 g/10 ml),

and the average particle size from DLS measurements, the

generation rate of Ag NPs is calculated to be 1.52� 108

nanoparticles formed per pulse, which is equivalent to 7.8

mg/h in terms of the particles weight.

To determine the structure and distribution of the Ag

NPs, we use high-resolution TEM, shown in Fig. 5. The

TEM images (Figs. 5(a) and 5(b)) demonstrate the existence

of Ag NPs and indicate their uniform distribution. Fig. 5(a)

is the dark field TEM image, in which only the diffraction

signal from crystalline structures is collected. Since arsenic

sulfide is in an amorphous state, the bright spots in Fig. 5(a)

are the silver nanoparticles. The diffraction pattern from the

Ag NPs (Fig. 5(c), inset) exhibits hexagonal symmetry that

can be attributed to the basal plane of the hexagonal phase.

This agrees well with the literature where it has been shown

the hexagonal phase of silver stabilizes only in the nanocrys-

talline form for particles less than 30 nm.26 Fig. 5(d) shows a

high resolution TEM image of silver nanoparticles which

demonstrates a high degree of crystalline order. The spacing

between the lattice fringes is measured to be 2.5 A.

The TEM image from Fig. 5(a) is transformed into a

high contrast image, as shown in Fig. 5(b), which more

clearly shows the spacing distribution of nanoparticles. We

quantify uniformity and dispersion of the distribution using

the nearest neighbor index (NNI), a standard method in spa-

tial analysis that is used to determine the degree of spatial

dispersion in a population.27 In general, if the distribution of

the points is clustered together, the average distance between

nearest neighbors will be shorter than if the particles are

scattered throughout the sample. The NNI is defined as the

ratio of the average inter-point distance between nearest

neighbors �d to the expected value of the average inter-point

distance if the sample were randomly dispersed Eð�dÞ.28,29

The value of the NNI can range between the theoretical

extremes of 0 (where all points are at the same location) and

2.1419 (where points have a perfectly uniform distribu-

tion).28 The equations for these parameters are given by

NNI ¼�d

E �dð Þ; �d ¼

Xn

i¼1

di=n; E �dð Þ ¼ 0:5ffiffiffiffiffiffiffiffiA=n

p: (1)

For the data in Fig. 5(b), n¼ 265 is the total number of

particles, obtained using the “particles analysis” utilities

from Image J, and A¼ 2156� 2156 nm2 is the area of the

studied region. di is the distance from ith particle to its

nearest neighbor, determined by measuring all neighboring

distances and taking the minimum. �d is calculated to be

66.68 nm using ImageJ. Eð�dÞ is determined to be 66.21 nm.

The NNI is then readily evaluated to be 1.01, which reveals

a random dispersion.

FIG. 3. DLS data with normalized areas. The measurement data are repre-

sented by the circles, while the trendlines are fitting curves using Gaussian

models. As2S3 solution shows peaks around 8 nm, while the average diame-

ters of the silver particles in the 40-min solution are approximately 14 nm.

FIG. 4. EDX study of the film spin-coated using the 40-min solution. A sili-

con wafer used as the substrate leads to the Si peak. The relative weight of

silver is 1.3%.

FIG. 5. Typical transmission electron micrograph image of Ag NPs gener-

ated by laser ablation of a silver target in As2S3/propylamine solution. (a)

Dark field TEM image of silver nanoparticles uniformly distributed inside

As2S3 film. (b) Inverted high contrast image corresponding to (a). Black dots

are silver nanoparticles. (c) Electron diffraction patterns from a cluster of

Ag NPs of the hexagonal phase with colored lines added for eye guiding. (d)

A high resolution TEM image of silver nanoparticles which demonstrates a

high degree of crystalline order. The spacing between the lattice fringes is

measured to be 2.5 A.

261906-3 Lu et al. Appl. Phys. Lett. 105, 261906 (2014)

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128.112.142.131 On: Wed, 25 Feb 2015 21:24:55

Finally, to test if the calculated NNI is statistically dif-

ferent from that of a random process it is necessary to calcu-

late the standard normal deviate of the distribution (Z) using

Z ¼ ð�d � Eð�dÞÞ=r�d , where r�d ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi0:0683A=n2

pis the stand-

ard deviation of �d .29 If the value of Z is within [�1.96, 1.96]

the distribution of points is considered to be random at the

95% confidence level.29 Z is calculated to be 0.218. The

value of NNI and Z evidences a uniform spatial dispersion of

the particles.

In summary, we report the fabrication of uniformly dis-

persed Ag NP/chalcogenide glass heterogeneous material

prepared through a combined laser- and solution-based

process. We are able to obtain uniform distribution of nano-

particles in both the solution and thin film phases, which is

evidenced by the high resolution TEM measurement and the

NNI analysis. The clustering or agglomeration that is typi-

cally associated with solution based methods in nanopar-

ticles fabrication are avoided through this approach. We

believe the process is applicable to other metals and other

chalcogenide glass solutions.30,31 These materials have a

great potential for applications in diffractive elements and

sensing devices, particularly in cases where thick films uni-

formly doped by Ag NPs are crucial. Furthermore, they have

ability to be photo-responsive, and could be used for direct

writing, as well as recording of optical information, such as

holographic data storage.

We thank Joseph Buttacci for assistance with

experimental setup construction. We thank Gerald Poirier

and Yao-wen Yeh for assistance characterizing samples. We

gratefully acknowledge support by NSF through the

MIRTHE Center (Grant No. EEC-0540832), as well as

support from the Princeton-University of S~ao Paulo

partnership program. J.M.P.A. acknowledges the S~ao Paulo

research foundation for the financial support. N.Y.

acknowledges the partial support of the NSF-MRSEC

program through the Princeton Center for Complex

Materials (DMR-0819860).

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