Metallic Nanoparticles

An Investigation of the Electronic PropertiesThe Beginning of…

Joe PetrusWatLABs

University of Waterloo

Purpose

To grow nanoparticles of copper, silver and gold on hydrogen terminated silicon and determine how the particle size and density change the electronic properties of the surface.

Sample Preparation

Copper Deposition Silver Deposition Gold Deposition

Hydrogen Termination of Silicon Wafers

Hydrogen Termination

As per Louis’ recipe…2.5mm by 15mm p-type silicon wafers are put in acetone and sonicated for 10 minutes.Put in 1:1:5 hydrogen peroxide, amoniumhydroxide and water solution for 30 minutes + heat ~70deg C.Put in 1:1:5 hydrogen peroxide, hydrochloric acid and water solution for 30 minutes.Put in 2% HF for 10 minutes.Rinse between each step with millipore water.

Metal Deposition

Copper Solution Silver Solution Gold Solution• Standard Solution:

• 100 micro M Ag2SO4 + 0.1 M Perchlorate

• Hg / HgSO4 Reference Electrode

• Standard Solution:

• 100 micro M AuCl3 + 0.1M Sodium Sulfate

• Ag / AgCl Reference Electrode

• Standard Solution:

•100 micro M CuSO4+ 0.1 M Perchlorate

• Ag / AgCl Reference Electrode

Electrochemistry – Cell SetupThe cell is setup with a hydrogen terminated silicon wafer as the working electrode, a platinum counter electrode and the appropriate reference electrode.

Most of the electrochemical experiments were done on the CHI 660 station.

The counter electrode is flamed for 30 seconds and the solution is bubbled with N2 for 15 minutes prior to deposition.

CV CurvesUsed to discover potentials at which the metal exhibits a characteristic peak related to nucleation and growth. Use values around these peaks as a starting point for deposition using amperometric I-t.

Amperometric I-t

Used to deposit metallic nanoparticles on H-Siby selecting a potential to apply to the cell and running for a certain amount of time.

Double Pulse

The double pulse technique can be employed to grow particles of a more specific and more consistent size.

The first pulse is for a short period at a more negative potential. This causes nucleation sites to form.The second pulse is for a longer period at a less negative potential. This causes the nucleation sites to grow.This has been shown to have success with silver nanoparticles, M. Ueda et al. / Electrochimica Acta 48 (2002) 377-386.For example first pulse: -1.5V for 60ms, second pulse: -0.7V for 120s.But the SEM died before I could verify this or try it with my copper and gold.

Variables

In the deposition process there are many variables which can be tuned to optimize the deposition. The ones I tried are:

Metal concentrationPresence of electrolyteDeposition potentialDeposition time

Sample Analysis

Now the samples are ready for analysis. First they should be classified with the SEM then further analyzed using the ESCALAB 250.

XPS elemental analysis or cleaning UPS work function analysis

Beautiful Metal Nanoparticles Verified + Classified With SEM

SEM Imaging

The Leo SEM was used to characterize the nanoparticles formed by electrodeposition.Representative areas of each sample were captured at 10, 50, 100 and sometimes 150 KX using the SE2 and RBSD detector.EDAX data was collected to make sure what I was looking at was the expected metal.

Copper

Mean diameter of ~ 40nm, spread of 25nm to 60nm

Copper EDAX Data

Silver

Mean diameter of ~ 50nm, spread of 20nm to 70nm

Silver EDAX Data

Gold

Mean diameter of ~ 20nm, very small spread – too small to see with EDAX

RBSD Images

Copper Silver

RBSD Images

Gold

Copper Particle Size Trend

Particle Size vs. Deposition Time

010203040506070

0 50 100 150

Deposition Time / s

Part

icle

Siz

e / n

m

The size of the nanoparticles does not change much with deposition time, the number density rises instead.My silver and gold particles also experienced this.

Copper Particle Size TrendFor a deposition time of 1 second, the potential was varied and again I saw almost the same particle size though the coverage changed dramatically. For example –1.1V gave ~ 44nm mean and –0.21V gave ~ 43nm mean.Note that this must be studied more thoroughly before forming a conclusion, tests with silver and gold are lacking.

Deposition ProblemsCopper, silver and gold are easily deposited by using the amperometric I-t technique but the particle size is not so easily controlled.The double pulse technique has been shown to control the silver nanoparticle size at least. M. Ueda et al. / Electrochimica Acta 48 (2002) 377-386.I have tried this technique but the SEM died before I could verify whether or not it works for copper and gold as well.

XPS AnalysisXPS Analysis was done using the ESCALAB 250.Argon sputtering was done at a pressure of 1.5x10-8

mbar before Andy’s visit and 3x10-8 mbar prior to his visit.Analysis pressure varies from mid 10-10 mbar to low 10-9 mbar range.

XPS Analysis

XPS was used to determine the elemental composition of my samples as well as for sample cleaning.I did not experience extensive contamination problems as was Louis’ case.

UPS Analysis

The ESCALAB 250 was again used to make UPS measurements.A HeI UV lamp was used.Pressure was approximately 3x10-8 mbar.

UPS Analysis

UPS was used to determine the work function change.Initially the plan was to determine the work function change as a function of particle size – however, due to limited time and SEM downtime my recent work function measurements are more geared towards coverage.

Work Function Samples

As stated before the work function measurements are focusing more on coverage than particle size due to SEM downtime. The particles were all prepared in a similar way, using the solutions outlined previously and depositing at about –1.0V for 1s, 60s and 600s for each of Cu, Ag and Au.

Work Function ChangeCombining XPS with argon sputtering and UPS I was able to measure work function changes.XPS was used to monitor carbon levels as I sputtered the surface down using argon.UPS was used to measure the work function change by scanning samples with different coverage against a benchmark – clean H-Si.

Work Function Change

The work function of a surface is the energy required to transfer an electron from the surface to a point where it is no longer effected by the surface.What I am measuring is the work function change because the work function of the spectrometer is greater than the work function of my sample.

Work Function Change

Borrowed from JörgMichael Gottfried‘s dissertation.

Work Function ChangeSince the ϕsp > ϕs we have to apply a bias on the sample to make sure that the kinetic energy of the photoelectrons is greater than ϕsp - ϕs.This was done at voltages from –2V to –6V all of which seemed to provide the required energy but –2V.

-3 to -6 Volts AppliedSurvey

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CPS

20 10 0Binding Energy (eV)

This is the shift resulting from an increase of 1V each step, starting at –6V bias at the right. Almost exactly 1V between each – as it should be.

Sample CleaningIt was determined by taking a UPS spectrum before cleaning and after cleaning that a dirty, I.e. carbon contaminated samplewill have a very different spectrum than that of a clean sample.

Survey

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Survey

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Unclean Clean

XPS Cleaning Before and AfterSurvey

NameO 1sC 1sCu 2pSi 2p

Pos.533.00285.50933.50100.00

FWHM1.8561.6662.0101.413

Area0.50.10.20.2

At%29.84822.065

1.54946.538

O 1s

C 1s

Cu 2p

Si 2p

x 103

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1000 800 600 400 200 0Binding Energy (eV)

Copper before Ar sputtering

Survey

NameO 1sC 1sCu 2pSi 2p

Pos.533.00285.00933.5099.50

FWHM2.0132.1151.9321.522

Area0.40.00.30.3

At%27.382

2.9232.232

67.462

O 1

s

C 1

s

Cu 2

p

Si 2

p

x 103

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1000 800 600 400 200 0Binding Energy (eV)

Copper after Ar sputtering

Silver before Ar sputtering

Survey

NameO 1sC 1sSi 2pAg 3d

Pos.533.00285.00100.00368.50

FWHM1.8581.7611.3281.188

Area0.40.10.31.5

At%21.94518.63946.93912.477

x 104

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1000 800 600 400 200 0Binding Energy (eV)

Survey

NameO 1sC 1sSi 2pAg 3d

Pos.532.50285.0099.50

368.50

FWHM2.1322.1371.5691.269

Area0.30.00.42.1

At%16.5523.844

62.50617.098

O 1

s

C 1

s

Si 2

p

Ag 3

d

x 104

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Silver after Ar Sputtering

Survey

NameO 1sC 1sAu 4dSi 2p

Pos.533.00285.00335.0099.50

FWHM1.8561.6814.1271.485

Area0.40.20.70.4

At%17.86820.427

4.24157.464

x 104

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Gold before Ar sputtering

Survey

NameO 1sC 1sAu 4dSi 2p

Pos.532.50285.00335.50

84.00

FWHM2.0001.8654.1931.526

Area0.40.00.92.1

At%4.6511.1971.607

92.545

x 104

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Gold after Ar sputtering

Sample CleaningNotice that after sputtering the sample with Ar for more than a minute and a half the oxygen peak has barely decreased. This implies that the oxygen in the sample is not actually on the surface such as an oxide of the metal deposited but instead is in the form of SiO2 which means that our procedure for Sitermination may need to be adjusted. In fact you can see the SiO2 peak at 103 eV.

Ideal Hydrogen TerminationAs suggested by Y. J. Chabal, ideal hydrogen termination of Si occurs when etching the wafer with HF (49% in H2O) and modifying the pH of the solution to be basic (pH = 9 –10) by adding NH4F as a buffering agent + NH4OH to increase the pH.G. S. Higashi, Y. J. Chabel, G. W. Trucks and K. Raghavachari, Appl. Phys. Lett. 56(7).

Preliminary Work Function ResultsCopper UPS with 5V bias

0.000

200000.000

400000.000

600000.000

800000.000

1000000.000

1200000.000

1400000.000

-5.0000.0005.00010.00015.00020.00025.000

Binding Energy / eV

CPS

1 sec60 sec600 secH-Si

Copper Up Close

Here, purple is bare H-Si, lowest coverage is blue, mid is gold and highest is red.

Survey3

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15 14.5 14 13.5 13 12.5Binding Energy (eV)

Silver Up Close

Here red is bare H-Si, purple is highest coverage, blue is lowest coverage and gold is inbetween.

Survey4

x 104

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15.5 15 14.5 14 13.5 13 12.5 12 11.5 11Binding Energy (eV)

Gold Up Close

Here purple is bare H-Si, gold is high coverage, blue is mid and red is low coverage.

Survey1

x 105

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15 14 13 12 11 10Binding Energy (eV)

ConclusionsSamples will need to be more carefully prepared and analyzed to be able to know how the work function changes.Our hydrogen termination procedure needs to be tuned because even after lengthy sputtering oxygen remains.The amperometric I-t method of deposition is not very consistent and is probably not suitable for creating nanoparticles of sizes less than 20nm.

Future WorkClassify particles grown electrochemically with the SEM.Use the double pulse technique as described to grow better nanoparticles.Create smaller nanoparticles using different methods such as vapor deposition.Investigate better H-termination of Si – see G. S. Higashi, Y. J. Chabel, G. W. Trucks, K. Raghavachari, Appl. Phys. Lett. 56(7) and related papers.

Acknowledgements

Many thanks to all the people here at WatLABs, it has been great experience working here.Thanks also to NSERC for supporting this work.

ResourcesC. N. R. Rao, G. U. Kulkarni, P. John Thomas, and Peter P. Edwards, Chem. Eur. J. 2002, 8, No. 1.Jörg Michael Gottfried, Adsorption und Reaktion von Sauerstoff, Kohlenmonoxid und Kohlendioxid an einerAu(110)-(1x2)-Oberfläche.M. Ueda, H. Dietz, A. Anders, H. Kneppe, A. Meixner, W. Plieth, Electrochimica Acta 48 (2002) 377-386.G. Oskam, J. G. Long, A. Natarajan, P. C. Searson, J Phys. D: Appl. Phys. 31 (1998) 1927-1949.Chunxin Ji, Gerko Oskam, Peter C. Searson, J. Electrochem Soc, 148 (11) C746-C752.