Altering Plasmonic Nanoparticle Size Through Thermal 

Annealing for Improved Photovoltaic Devices 

by Rukshan Fernando, Fred Semendy, and Priyalal Wijewarnasuriya 

ARL-TR-5895 January 2012 

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Army Research Laboratory 

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ARL-TR-5895 January 2012 

Altering Plasmonic Nanoparticle Size Through Thermal 

Annealing for Improved Photovoltaic Devices 

Rukshan Fernando 

University of California Irvine, CA 92697

Fred Semendy and Priyalal Wijewarnasuriya 

Sensors and Electron Devices Directorate, ARL 

Approved for public release; distribution unlimited. 
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Altering Plasmonic Nanoparticle Size Through Thermal Annealing for 

Improved Photovoltaic Devices

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6. AUTHOR(S)

Rukshan Fernando, Fred Semendy, and Priyalal Wijewarnasuriya

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U.S. Army Research Laboratory

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14. ABSTRACT

We created gold (Au) and silver (Ag) nanoparticles by e-beam deposition on an n-type silicon substrate followed by rapid 

thermal annealing (RTA) under nitrogen gas. Thermal annealing was performed in two ways: first by varying temperature for 

a constant time and second by varying time at a constant temperature. RTA under the right eutectic temperature, which varies 

metal to metal, should create nano-metal particles on the substrate. These nanoparticles were characterized by optical 

spectroscopy and atomic force microscopy (AFM). Results from the optical measurements provide information on the 

reflective and absorptive characteristics of Au and Ag nanoparticles. AFM provided additional information about the surface 

properties including size, density distribution, and other relevant values. Results are reported here and they indicate the nature 

of the Au and Ag nanoparticles by their optical and surface characterization. We propose that the absorption properties can be 

obtained in solar cells by exploiting the plasmonic behavior of these nanoparticles.

15. SUBJECT TERMS

Plasmonic nanoparticles, thermal annealing, photovoltaic devices

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Fred Semendy

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iii

Contents 

List of Figures iv

List of Tables iv

Acknowledgments v

1. Introduction 1

2. Experimental Procedures 3

3. Results and Discussion 7

4. Conclusion 10

5. References 11

List of Symbols, Abbreviations, and Acronyms 12

Distribution List 13
iv

List of Figures 

Figure 1. Plasmonic solar cell using metal nanoparticles on Si substrate. .....................................1

Figure 2. (a) CHA E-beam evaporation machine procedure and (b) deposition process for 

20-Å Au/Ag particle formation on Si surface, deposited at 1.7 x10–6 Torr at 0.3Å/s. ..............4

Figure 3. RTA process used for experiment. ..................................................................................5

Figure 4. Picture of Perkin Elmer Lambda 950 UV-Vis spectrometer used for optical 

measurements. ............................................................................................................................7

Figure 5. (a) Reflection of Ag with time variable samples and (b) absorption of Ag with 

temperature variable samples. ....................................................................................................8

Figure 6. AFM pictures of step height analysis of Au and Ag samples. ........................................9

Figure 7. AFM surface plots of samples with corresponding rms values. ......................................9

List of Tables 

Table 1. With the eutectic temperature for Au being 389 °C and 205 °C for Ag the following 

samples were annealed accordingly. ..........................................................................................6

Table 2. Other nanoparticle size measurements acquired from AFM analysis. ...........................10
v

Acknowledgments 

Rukshan Fernando wishes to acknowledge the assistance of Dr. Fred Semendy, Dr. Priyalal 

Wijewarnasuriya, and Gregory Meissner of the U.S. Army Research Laboratory. 
vi 

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1

1. Introduction 

Photovoltaics is a method of generating electrical power by converting solar radiation into direct 

current electricity using semiconductors that use the photovoltaic effect (1). Thin-film solar cells 

are a new design approach for photovoltaic devices. Photovoltaic cells are made of special 

materials called semiconductors, such as silicon (Si), which is currently the most commonly used

material. Basically, when light strikes a cell, a certain portion of it is absorbed within the 

semiconductor material, which means that the energy of the absorbed light is transferred to the 

semiconductor itself. This process is shown in figure 1. The transferred energy knocks the

electrons loose, allowing them to flow freely. Photovoltaic cells have one or more electric fields 

that act to force electrons freed by light absorption to flow in a certain direction (2). This flow of 

electrons is called a current, and by placing metal contacts on the top and bottom of the 

photovoltaic cell, we can draw that current off for external use in which there are thousands of 

applications in the U.S. Army. 

Figure 1. Plasmonic solar cell using metal nanoparticles on Si substrate.

One problem is that pure crystalline Si is a poor conductor of electricity because none of the 

electrons are free to move around (3). To help with this issue, the Si in a solar cell has impurities. 

The process of purposely adding impurities is called doping, in which the resulting Si is called ntype because of the prevalence of free electrons. N-type doped Si, which is used for this 

experiment, is a much better conductor than pure Si (4). However, in this experiment, we are 

dealing with thin-film solar cells and the biggest problem for thin-film solar cells is that they 

don’t absorb as much light as the current solar cells do. 

Methods for trapping light on the surface or in the solar cell are crucial in order to make thin-film 

solar cells. One method that has been explored over the past few years is to scatter light using 

metal nanoparticles excited at their surface plasmon resonance (5). This allows light to be 

absorbed more directly without the relatively thick additional layer required in other types of 
2

thin-film solar cells. In recent years, plasmonic nanoparticles closely adhered to absorbing 

semiconductors have been used to enhance absorption in ultrathin-film solar cells (6). When light 

hits these metal nanoparticles at their surface plasmon resonance, the light is scattered in many 

different directions. This allows light to travel along the solar cell and bounce between the 

substrate and the metal nanoparticles, enabling the solar cells to absorb more light (7). 

Observing the relationship between the size of the nanoparticles and the detection or absorption 

of the particles is an important concept in plasmonic research. In this experiment we deposited 

metal nanoparticles of gold (Au) and silver (Ag) on the top surface of a thin-film solar cell, and 

focused on characterizing the optical properties of the plasmonic nanoparticles and determining 

how much the absorption sensitively depends on the nanoparticles size and shape and whether

this can be altered by thermal annealing.

In this experiment, metal particles were deposited from CHA E-beam evaporation. The samples 

were then thermally annealed using rapid thermal annealing (RTA) to create nanoparticles. We 

chose RTA over other methods for creating nanoparticles, such as inert-gas condensation and 

chemical synthesis. Inert-gas condensation is frequently used to make nanoparticles from metals 

with low melting points. The metal is vaporized in a vacuum chamber and then super-cooled 

with an inert gas stream. The super-cooled metal vapor then condenses into nanometer-sized 

particles, which can be entrained in the inert gas stream and deposited onto a substrate. Chemical 

synthesis, specifically, sol-gel processing, is a wet chemical synthesis process used to generate 

nanoparticles by gelation, precipitation, and hydrothermal treatment. The advantage to creating 

nanoparticles using RTS is that it is much faster than any other method, including inert-gas 

condensation, and thus, due to time constraints within the research effort, we deemed it the best 

choice. 

We created these nanoparticles, grown on Si substrates, for testing and examination. Si is the 

chosen best material for the photovoltaic application substrates because of its low cost, 

abundance in nature, long-term stability, nontoxicity, and well-established technology. 

The absorption of the applied light onto the nanoparticles grown on the substrate depends on 

each particle’s size. Metallic particles that are much smaller than that of the wavelength of light 

usually absorb more. Many researchers have looked into this new development, one of whom 

being S. Pillai (8). Pillai has done research in which Ag nanoparticles were deposited onto a Si 

wafer on an insulator. He has demonstrated absorption and emission enhancement from Si solar 

cells by using the nanoscale properties of metals. He targeted enhancement close to the bandgap 

of Si, where absorption is weak. For his experiment, metal nanoparticles were deposited by 

thermal evaporation of thin layers, which was then followed by thermal annealing. The Ag

nanoparticles shape was tuned by a thermal annealing at temperatures well below the melting 

point of Ag. Then due to surface tension, the particles merge together to form islands. This 

process is one of the simplest ways to deposit metal nanoparticles onto a substrate. After running 

the experiment it was concluded that for front surface application, smaller metal particles provide 
3

maximum enhancement in the visible (Vis) as well as the near-infrared (NIR), but larger 

particles would be more beneficial from both thin and thick light-emitting diodes (LEDs) (9). 

Thin-film solar cells have the potential to significantly decrease the cost of photovoltaic devices. 

Light trapping is very critical in thin-film crystalline Si solar cells in order to increase light 

absorption and cell efficiency (10). High efficiency solar cell design needs optically thick 

photovoltaic layers to give nearly complete light absorption, as well as minority carrier diffusion 

lengths many times the material thickness to allow efficient carrier collection (11).

2. Experimental Procedures 

The Au and Ag layers were deposited onto double side polished (DSP) <100> Si substrates using 

a CHA E-beam evaporator. The two separate Si wafer substrates used for each metal deposition

were also n-type, meaning they had negative-type doping. Electron beam evaporation is a form 

of physical vapor deposition in which a target anode is bombarded with an electron beam given 

off by a charged tungsten filament under high vacuum. The electron beam causes atoms from the 

target to transform into the gaseous phase. These atoms then precipitate into solid form, coating 

everything that is in the vacuum chamber with a thin layer of the anode material. The E-beam 

evaporation process is shown in figure 2a. The goal was to deposit only 20 Å of Au and 20 Å of 

Ag onto the other, because this would ensure a more uniform layer of particles. The process of 

depositing metal nanoparticles onto the Si substrate is shown in figure 2b. In order to work 

properly with best results, the pressure in the CHA evaporator was maintained below 

1.7x10–6Torr. The Au and Ag metal particles were deposited separately and evaporated onto the 

substrate at a rate of 0.3 Å/s to ensure an even layer of particles on the surface. All of the 

deposition procedures were done in the U.S. Army Research Laboratory (ARL) cleanroom.
4

Figure 2. (a) CHA E-beam evaporation machine procedure and 

(b) deposition process for 20-Å Au/Ag particle formation 

on Si surface, deposited at 1.7 x10–6 Torr at 0.3Å/s. 

The Au and Ag deposited wafers were then cleaved in which the goal was to have 12 identicalsized samples of Si-Au and 12 identical-sized samples of Si-Ag. First the Si substrate was 

measured and lined up to be to cut into identical 2x2 cm pieces with a diamond tip pen. Then the 

Si wafer was simply cleaved by snapping and breaking the pieces along the inscribed lines. The 

samples were then cleaned using basic lab cleaning procedures. First, each sample was washed 

with acetone and then with isopropyl alcohol (IPA). Then, the samples were rinsed off with deionized water and finally blow dried with a nitrogen gun. Both the Si-Au and Si-Ag were cleaved 

and cleaned in the same manner. 

Silicon Substrate

Au & Ag Deposition
5

Samples were then annealed in the rapid thermal annealer. RTA is a process used in 

semiconductor device fabrication, which consists of heating a single wafer at a time in order to 

affect its electrical properties. Unique heat treatments are designed for different effects. The 

RTA process is performed by equipment that heats a single wafer at a time using lamp-based 

heating and a hot plate that a wafer is brought near. The wafer is heated by lamps located on top 

and below the sample holder and the pyrometer is used to determine the temperature of the 

sample’s surface, which is shown in figure 3. Unlike furnace anneals, they are short in duration, 

processing each wafer in several minutes. For this experiment, RTA was used to create the 

nanoparticles only, but wafers can also be heated to activate dopants, change film-to-film or 

film-to-wafer substrate interfaces, densify deposited films, change states of grown films, and 

even repair damage from ion implantation. During cooling, however, wafer temperatures must be 

brought down slowly so they do not break due to thermal shock. To achieve short-time 

annealing, the trade off is made in temperature and process uniformity, temperature measurement 

and control, and wafer stress as well as throughput. One main problem with the RTA process is 

keeping the wafer at a constant temperature. The annealing temperature usually shoots up 

10–20 °C above the needed temperature during the process, which affects the consistency of the 

resulted samples. To avoid this problem, a trial run was conducted with a blank sample in 

between each process and the power of the machine was adjusted according to the needed 

temperature.

 

Figure 3. RTA process used for experiment. 

Each 2x2 cm sample was annealed at a different temperature and for a different amount of time 

for each sample. Annealing under the right eutectic temperature for Au, which is 389 °C, and Ag, 

which is 210 °C, should create nano-metal particles on the substrate. Six samples of Au were 

annealed from 250 to 500 °C with a temperature difference of 50 °C in between each sample at a 
6

constant time of 30 min. Then a separate six samples of Au were annealed from 10 to 110 min 

with a time difference of 20 min in between each annealing process at a constant temperature of 

450 °C. Five samples of Ag were annealed from 200 to 400 °C with a difference of 50 °C in 

between each annealing at a constant time of 50 min. Finally, a separate five samples of Ag 

were annealed from 30 to 70 min with a time difference of 10 min in between each annealing 

process at constant temperature of 250 °C. The process for each individual sample is shown in 

table 1. To start the RTA process, nitrogen gas is turned on to 4 psi for lamp cooling and RTA is 

switched on. The Set II temperature, which is the temperature at which the process begins, is 

always set to150 °C and the Set I temperature is set to the maximum temperature that is needed 

for the specific sample. By placing each sample metal side up onto the annealing wafer and 

setting the timer to the needed time, the samples were annealed. 

Table 1. With the eutectic temperature for Au being 389 °C and 205 °C for Ag the following samples were annealed 

accordingly. 

 

The annealed samples were used for optical measurements, which include the reflection, 

transmission, and absorption of the samples. To test the performance of the nanoparticles on the 

substrate, the optical measurements were obtained by using the Perkin Elmer 950 ultraviolet 

(UV)-Vis-NIR spectrometer. For testing the transmission of each sample, the auto-zero, or a 

blank, was done for reference and to calibrate the spectrometer to ensure maximum accuracy 

before testing each sample. After performing the auto-zero, the samples were tested. When 

making transmission measurements, the spectrometer quantitatively compares the fraction of 

light that passed through the reference, or blank test, and the test sample. For reflectance 

measurements, the auto-zero or reference is calculated by a 100% reflective mirror. Then, the 

spectrometer quantitatively compares the fraction of light that reflects from the reference mirror

and the test samples. Figure 4 shows the process of how the spectrometer collects data for 

transmission and reflection. The transmission and reflection data were measured between 

wavelengths of 400–1200 nm, using the spectrometer by positioning each sample to the holder.
7

Figure 4. Picture of Perkin Elmer Lambda 950 UV-Vis spectrometer used for optical measurements. 

The samples were then examined using atomic force microscopy (AFM), which was used to 

examine the physical properties of the nanoparticles. AFM consists of a cantilever with a sharp 

tip at its end that is used to scan the surface of the substrate. The cantilever is typically Si or 

silicon nitride with a tip radius of curvature in order of nanometers. When the tip is brought onto 

a sample surface, forces between the tip and the sample lead to a deflection of the cantilever 

according to Hooke’s law. AFM has many advantages over the scanning electron microscope 

(SEM). Unlike the SEM, which provides a two-dimensional projection of a sample, AFM can 

provide a three-dimensional surface profile. 

3. Results and Discussion 

The data collected from the annealed samples were examined with the spectrometer and were 

measured by shooting light at each sample between wavelengths of 400–1200 nm. The 

spectrometer gives data between the wavelength of the light that is applied to the surface and the 

effect it has with the surface of each sample. Graphs for each time-dependent and temperaturedependent sample were plotted using Microsoft Excel. After calculating the reflection and 

transmission of each sample and graphing the data with respect to the wavelength, the absorption 

was calculated by using the following formula and were graphed: 
8

(1) 

The reflection and absorption data are plotted in figure 5a and b. According to the graph data, as 

the size of the Ag nanoparticles increased, the reflection of each sample also increased but for 

also with large particle size there tends to be a reduction in absorption at shorter wavelengths. 

Also, there is an increase in absorption with large particles at longer wavelengths. From the 

graphs, smaller particles also show to be less reflective than the larger particles. 

 

Figure 5. (a) Reflection of Ag with time variable samples and (b) absorption of Ag with temperature variable 

samples. 

The samples were then examined using AFM, which was used to examine the physical properties 

of the nanoparticles. Samples A1, A3, A5, B1, and B5 from the Ag samples and samples A1 and

A3 from Au were chosen to be used for AFM. However, first, each sample had to be fit into 

AFM device and thus were measured, lined up, and cut into identical 1x1 cm pieces with a 

diamond tip pen. Then, the samples were simply cleaved by snapping and breaking the pieces 

along the inscribed lines. After that, they were cleaned using basic lab cleaning procedures. 

The AFM procedure helps to see the actual size of each nanoparticle and make a relationship 

between the nanoparticles size and the data from the spectrometer. AFM can analyze many types 

of data such as the step height, bearing depth, depth, grain size, particle analysis, roughness, and 

section analysis of each sample. Of the measurements that AFM can provide, step height analysis 

is very important, because it provides information about the height of sample. The step height 

measurements of each sample are shown in figure 6. This is necessary because the curvature of 

the substrate might well be sufficient to prevent the use of the simple least-squares line fit. When 

measured, the step and substrate areas are treated as line segments, allowing the curvature of the 

substrate to be removed, resulting in a straight-line representation of the substrate. The step 

heights are calculated from this line in the areas adjacent to each step.
9

Sample Step height Sample Step height Sample Step height

Ag A1 Ag B1 Au A3

Ag A3 Ag B5

Figure 6. AFM pictures of step height analysis of Au and Ag samples. 

The scanned area was 2 μm x 2 μm and the root mean squared (rms) values were obtained. 

Figure 7 shows the AFM surface plots of the samples that were analyzed. The first one are Ag

nanoparticles on Si annealed at 200 °C with a density of 3.7 x 1010 per cm2. Sample Ag A3 has 

been annealed at 300 °C, which is higher than the 200 °C at which sample Ag A1 was annealed. 

Sample Ag A1 exhibits smaller-sized nanoparticles of Ag than sample Ag A3. The rms values of 

each sample also noticeably increases as the temperature and time at which they are annealed 

increases. 

Sample AFM 

Roughness

rms Value Sample AFM 

Roughness

rms Value Sample AFM 

Roughness

rms Value

Ag A1 2.75 nm Ag B1 3.64 nm Au A3 1.075 nm

Ag A3 3.067 nm Ag B5 3.687 nm

Figure 7. AFM surface plots of samples with corresponding rms values. 

Table 2 lists the bearing depth, grain height, and particle height of the scanned samples. The 

bearing depth, which is presented by average height in bearing area, is seen here as increasing for 

the annealed samples. The grain height, which measures the height of each individual grain, is 

also shown as growing for the annealed samples. The particle height, which is a measurement of 
10

the height of each, also increases. A trend in increasing value is seen in the results as the 

annealing process changes for each sample. The more a sample is annealed, whether it is 

increase of the temperature or the annealed time, the physical property values also increase.

Table 2. Other nanoparticle size measurements acquired from AFM analysis. 

Sample Bearing Depth

(nm)

Grain Height

(nm)

Particle Height

(nm)

Ag A1 9.261 2.378 13.824

Ag A3 16.197 12.293 15.916

Ag B1 18.309 11.339 13.69

Ag B5 22.845 15.019 25.294

Au A3 4.095 2.712 5.235

4. Conclusion 

We investigated the optical and physical properties of Au and Ag nanoparticles grown on a Si 

substrate. The reflection, transmission, and absorption of Au and Ag nanoparticles were 

deposited onto Si wafers that were altered by different processes of thermal annealing. The size 

of the nanoparticles on each sample changed in each different case. It is evident that as the 

temperature and time, at which the thermal annealing occurred, the Au and Ag nanoparticle grew 

larger in size and the reflection, transmission and absorption also changed as the particles grew. 

The obtained results show that for front surface application, smaller metal particles provide 

maximum enhancement for visible detection. With large particle size, there tends to be a 

reduction in absorption at shorter wavelengths; however, there is an increase in absorption with 

large particles at longer wavelengths. From the graphs, the larger particles were also shown to be 

more reflective than the smaller particles. 

Through this work, we have shown that metal nanoparticles created by thermal annealing can 

change the physical and optical properties of the nanoparticles. Enhancing these nanoparticles 

can help to improve the conversion efficiency of solar cells if metal nanoparticles are included in 

the photovoltaic devices. 
11

5. References 

1. Nakayama, K.; Tanabe K.; Atwater, H. Plasmonic Nanoparticle Enhanced Light Absorption 

in GaAs Solar Cells. Journal of Appl. Physics 2008, 93, 121904-1.

2. Atwater, H.; Polman, A. Plasmonics for Improved Photovoltaic Devices. Nature Material: 

Review Article 2010, 205–208.

3. Pillai, S.; Catchpole, K. R.; Trupke, T.; Green, M. A. Surface Plasmon Enhanced Silicon 

Solar Cells. Applied Physics 2007, 101, 1–6.

4. Popok, V. N.; Gromov, A. V.; Nuzhdin, V. I.; Stepanov, A. L. Optical and AFM Study of 

Ion-synthesized Silver Nanoparticles in Thin Surface Layers of SiO2 Glass. Journal of NonCrystalline Solids 2010, 35, 1259.

5. Chen, F.; Johnston, R. L. Plasmonic Properties of Silver Nanoparticles on Two Substrates. 

Springer Science: Plasmonics 2009, 147–148.

6. Okumu, J.; Dahmen, C.; Luysberg, M.; Wuttig, M. Photochromic Silver Nanoparticles 

Fabricated by Sputter Deposition. Journal of Appl. Physics 2005, 97, 1–2.

7. Ferry, V.; Verschuuren, M.; Li, H.; Verhagen, E.; Atwater, H.; Polman, A.; Walters, R. 

Light Trapping in Ultrathin Plasmonic Solar Cells. Optics Express 2010, 18 (7), 5–7.

8. Lojek. B. Early History of Rapid Thermal Processing. ATMEL Corporation 2005, 3–10.

9. Beck, F. J.; Mokkapati, S.; Catchpole, K. R. Plasmonic Light-trapping for Si Solar Cells 

using Self-assembled, Ag Nanoparticles. Progress in Photovoltaics. 2010, 18 (7), 501–502.
12

List of Symbols, Abbreviations, and Acronyms 

%R percent reflection 

%T percent transmission 

AFM atomic force microscopy 

Ag silver 

Au gold 

DSP double sided polish 

IPA isopropyl alcohol 

NIR near infrared 

Vis visible 

UV ultraviolet 

nm nanometers 

n-type negative type 

rms root mean squared 

RTA rapid thermal annealing 

SEM scanning electron microscope

Si silicon 
13

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